[asid61]

(T1 + T2)/(DP*2) + C should be the distance between two gears.
T1 is the teeth on gear 1. T2 is the teeth on gear 2. DP is the diametrical pitch (usually 20 for FRC gears). C is the clearance, an added distance between gears to ensure that manufacturing tolerances don't force the gears to close to each other. Two gears must have the same DP to mesh properly.
Basically, you want the "pitch diameter" of two gears to be tangent, with a spacing of C between the two diameters. The pitch diameter is equal to T/DP, where T is the teeth on the gear and DP is the pitch. Just draw a circle, plus that in for diameter, and add C to the resulting number. Do the same for t your second gear and make the two circles tangent.
On a part to be milled or CNC'd, I use a C of 0.000". For waterjetted stuff I am starting to use 0.003" in case the holes are too close and I don't want to mill it as well.