Well, there can and must be no question of an unequivocal answer to this among advanced students, though naturally one tends to simplify in order to explain things to beginners and declare the ideal 45-degree turn to be the standard procedure.
However, the highest principle in WingTsun is to behave naturally and adapt yourself to the actions of the opponent. This means there can be no rigid instruction such as "Do a 90-degree turn!" or "Never turn by more than 45 degrees!" among experts. An instruction like this would be a serious simplification, turning the glorious, scientific system which Great Grandmaster Leung Ting has passed on to us into a mere method which would certainly be quicker and easier to learn, but would no longer retain the riches it contains. Nonetheless it is literally "in my hands" to counter the still weak attack of my opponent – after all, it is only just beginning – with an early reaction of my own, which limits his forward pressure and therefore his power to turn me. For the less far I have to turn, the faster I can launch a (counter) attack, therefore it follows that the smallest adequate turn is the best.

Let me start by pointing out that according to GGM Leung Ting the word "turn" does not refer to the upper body but to the stance, i.e. to the feet.
Strictly speaking we should therefore be talking about a "stance turn".
The upper body should not be rotated around its centre axis, a misunderstanding common amongst students and unfortunately also amongst some advanced WT people. There may be Wing Chun or Ving Tsun styles not derived from Great Grandmaster Leung Ting which use such theories, and within their own concept they may be satisfied with them, but we in Leung Ting WingTsun take a completely different view.
Successful WingTsun is the result of rigorously adhering to numerous behaviour patterns performed simultaneously or in sequence.
Its efficiency only comes from the sum of all these details.
In fact what we generally refer to as a "turn" does not involve "actively" rotating or throwing round the upper body, but rather shifting the bodyweight from one side to the other so that the opponent's attack does not strike home and he cannot compromise our balance.
This so-called "turn" or rather "shifting of bodyweight" is "passive", i.e. I do not really execute a turn myself, but allow the opponent to "turn" or "shift" me. This means that at this stage I do not turn myself, but allow the opponent to move me. The fact that I offer no resistance but operate on the revolving-door principle enables me to maintain my balance according to the motto: "Those who give themselves over prevail".
And since I do not "turn" actively it is also logical that it is not I who decides how far I turn, i.e. by how many degrees. After all I am not turning, I am being turned. Please be sure to note this small but oh so vital difference at all times.
The angle at which my upper body presents itself to the opponent corresponds to his attack. Which shows that I am following the WT maxim: "When turning it is first the arm that moves, then the stance".
As an example, let's take a punch with attacking step by the opponent which I intend to counter with an inside punch. At this point the opponent proves stronger than I, which causes me to give way (maxim: "If the opponent's attack is more powerful, give way!"). What my Si-Fu would call my "optimistic" attack, which I was unable to complete and which was therefore "frustrated", adapts and changes shape into what we might at a certain point call the ideal "Bong-Sao" if somebody were to take a snapshot of it at that moment.

When the arm has completed its deformation it will ideally not move any further, but instead the body will continue to give way. This follows the maxim of the so-called "immovable elbow": "When the body moves the arm or elbow does not move".
During the process my angle versus the opponent depends on the extent to which he has penetrated my stance, and not so much on any conscious decision by me which would make the entire sequence an act of will requiring much more preparation and delivery potential than a tactile reaction.
If the attacker comes to a stop with his front foot in front of me he will be unable to move my upper body, or only by very little, but if he is a WT expert who penetrates deeply into my stance by using the "Bik Bo Tip Dar" deep-penetration technique, he will move me by several degrees.
It follows from the above that I must not turn actively or have a certain number of degrees fixed in my brain. Anybody who turns actively is always in danger of overdoing the turn, which can have disastrous results if the attacker knows his business.
Too much is always wrong, as we already know from the Jung Yung of Confucius (The Doctrine of the Mean), which was the philosophy followed by the late Great Grandmaster Yip Man. Most of us are familiar with the famous inscriptions at the temple in Delphi which are ascribed to the seven wise ones:
"Know yourself" (gnothi seauton) is one, but the second and no less important inscription is "Nothing to excess!" (meden agan). I am not alone in thinking that all "sin" is due to ignoring this particular admonition in various ways.
If we turn actively, and therefore always too little or too much, it leads to disaster. But if I passively allow myself to be turned my angle will precisely correspond to the force and penetration of the opponent, which means that my centre-line – in this case represented by my shoulder – will be correctly angled towards him.
The really important thing is the centre-line, by which I mean mine! It must always be directed at the opponent. If my centre-line, i.e. the "shortest straight line between him and me", is not directed at the opponent but past him, then I am leaving him the shortest line from himself to me and am at the mercy of his attack along his centre-line. In this connection please remember the adage: "In cases where I am turned I form the centre-line with my shoulder".

Neither do I use the Tan-Sao from the 3rd or 6th set of the SNT form when I am turned, but rather the crossed Tan-Sao from the 1st set, with the fingertips of the Tan-Sao aimed at the opponent.
Let me also point out that the Tan-Sao in the 1st set of the Wooden Dummy form, as handed down to us by the late Great Grandmaster Yip Man on page 17 of the book "116 WingTsun Wooden Dummy Techniques" (photos 4 and 7), is quite rightly not the crossed Tan-Sao with the elbows held outwards. For in this case there is no "turn", but rather penetration using the "jamming step". Nonetheless, when using this technique (knowing as I do the history of the jamming step, which was formerly a simple and therefore fast turn) I would not regard it as an error to have the elbow outwards as in the crossed Tan-Sao to achieve a specific aim in certain cases.
Please also note the position of the late GGM Yip Man's feet ("plum-blossom steps"), as here too various theories are propounded which confuse many students. But that is another subject for a later, more in-depth editorial.

Conclusion:
As a teacher one should beware of giving all too precise instructions concerning the exact position of the feet, angles etc., as this tends to suggest that one has only a limited knowledge.
Only those who know little will be tempted to make generalised statements. Those who really know a great deal know that they know nothing, and would rather bite their tongues off than proudly assert their present, personal, second-hand or out-of-context interpretation as the ONLY and undying truth.
It is always right to be suspicious whenever an instructor who teaches something as complex as our WT claims to "know" something, for in every case WT depends on many, many circumstances and is not a lifeless body of knowledge that can be analysed under a microscope.

Special greetings

This month I would like to extend greetings to our member Bernd Aretz (12.10.36), who is probably the oldest WT Technician in the German-speaking countries. According to his instructor Oliver Hagenau he suffered a severe stroke some five weeks ago and lay in a coma for one night.
He is now responding to treatment and able to speak reasonably well again, though the left side of his body is still paralysed.
His instructor is optimistic that Bernd will marshal all his willpower to combat this illness.
Just how strong-willed Bernd Aretz is becomes clear from the fact that he has already asked Oliver Hagenau for a training schedule which will enable him to reach his next goal, the 2nd Technician.
Bernd is an example to us all, and we should support him by thinking of him and wishing him a speedy recovery. Please send any emails via oliver.hagenau@surfeu.de, fax no. +49 2166/127419, or write directly to Bernd Aretz, Karl-Platz-Straße 29, D – 41812 Erkelenz.