Switched to HEI

Unless you want to spend a lot of time and money with a four gas exhaust analyzer and a dyno, the only thing you can do is experiment with gap settings that give you the performance you want. Different engines want different settings, an open chamber head may not want the same gap as a closed chamber high squish head. Cylinder pressure greatly affects the gap, the more pressure the less gap it'll tolerate. RustyRatRod is right, even GM spec'd their gaps from .025 to .120 depending on what worked best for each engine.