My instructor taught me a very simple "clock code" method for working out drift & GS on un/p diversions, i.e. :
Wind angle <15 deg off track, no drift & full wind speed effect on GS
Wind angle <30 deg off track, half max. drift & half wind speed
Wind angle <45 deg off track, 3/4 max. drift & 1/4 wind speed
Wind angle 60-90 deg off track, max. drift & no wind speed effect on G/S
Very generic and simple but works for me
Smithy