[asid61]

I heard 0.001" is fine for clearance, but it really depends on who's machining it and how good you tolerance it.
The formula I use for gearboxes is (T1 + T2)/(P*2),
where T1 is the first gear teeth, T2 is the second gear teeth, and P is the pitch (usually 20 for FRC). Then I add 0.002" because I manual mill gearbox plates. Next year we're sending them out though.
Make sure that your cim mounting is really good. I add 0.002" for the cim gear to the first gear, because the cim does not press fit into the plate like a bearing. Rather, it is a very close fit with about 0.002" of clearance (diameter).