In FRC a lot of people use the following formula to calculate gear center to center dimensions:
C to C = ((N1 + N2) / (2 * DP)) + .003
N1 and N2 are the number of teeth in the two gears you're meshing, and DP is the diametrical pitch of the gears (which in your case is 20).
See this thread for more info:
http://www.chiefdelphi.com/forums/sh...ad.php?t=95413