Originally Posted by drauk
If you can do the trig in your head you'd have got answers of 25%, 50%, 70% and 86% respectively, so you can see it's a pretty good approximation.
That is the kernel of it all. In other words think of the shape of a sine wave, that will tell you roughly how much value to apply to the actual crosswind according to its angle.
One of the problems with flying tuition, IMO, is the plethora of little rules of thumb and mnemonics. Its considerably easier I think to equip oneself with the basic mathematical thinking to produce reasonable estimates, rather than try to learn which of the rules of thumb, etc., (which might be alien to your personal way of thinking) is supposed to apply in the particular case.