I've allways used this rule of thumb. When making baked goods such as cookies or brownies, I allways use about 2.5 times what it would ususally take to get me high (if the batch is getting split 3 ways and it takes a gram per person to normally get baked, I would use 7.5 grams.
When making stuff like firecrackers or whatnot, 1.5 x the normal dose is usually enough.

I also never eat anything else right before 'dosing'. I'm not sure if it does anything, but I like my stomach to be mostly empty before starting.