Depends how accurate you want it and whether you fly 'standard SEP training aircraft' - if you do, and you take into account
1- you are only using a forecast, and
2- forecast is for a set time at that
3- your accuracy of keeping a heading,
then I'd suggest 3° of max drift per 5 knots of wind might not be too inaccurate. (assumes aircraft cruises around the 85 - 115 kt range). so 20kt wind = 12° of max drift for these type of aircraft.