Max drift = 60/TAS*windspeed.
This comes from the 1-in-60 rule - an approximation which says that if a right-angled triangle has a long side of around 60 units, the length of the short side will be approximately the same as the angle oposite that short side. This approximation works very well for smallish angles.
Unfortunately, I can't think of an easy way of explaining how to get from the 1-in-60 rule to the max drift forumula without using diagrams.
Since you generally tend to fly at one particular TAS in your particular aircraft, you can simplify the equation even further for your favourite aircraft. So, for example, if your aircraft generally cruises at 90kt:
Max drift = 60/90*windspeed = 2/3 windspeed
Next - remember that this max drift will only apply when the wind is between 60 and 120 degrees from your side. For smaller angles, use the clock code (that's the same rule you use for working out crosswinds on a runway for takeoff and landing) applied to the max drift to work out the drift for your heading.
FFF
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