there is on of the guys who frequents the free forum who created a sailcalculator. insert weight and wind and you get a sailrange. for beginners it's a good indicator. Usually when i'm unsure i look at what the others are sailing. Just try to find someone that has a kit that would be simular to yours and make the best copy you can think of. but in the end, it's pure empirical science. Like ken sais, too many factors.
Haiko, AKA crazychemical