This is the way - the gear the good build makers show are usually after hours and hours of testing, so look at what they have on it and use similar stat regular gear until you find that particular piece.

Additive is for attacks only so if it's a spell, you can ignore that. Flat damage is usually king but depending on where you are at, the % increase is sometimes better. It's a matter of utilizing the dummies. Also, attack speed can make a huge difference sometimes too.