The functional diameter for center-to-center calculations is the pitch diameter. This diameter is located in roughly the middle of the height of the gear teeth. It is often given as a dimensional property of the gear, or can be calculated from other properties such as the pitch, no. of teeth, diametral pitch, etc.
For a gear pair, add the pitch radii (half the pitch dia.) of both gears to find the center-to-center distance.
Andy-Mark supplies the diametral pitch (they call it the "diametric" pitch) for their gears, which is the number of teeth per inch of pitch diameter. Divide the number of teeth by the diametral pitch to find the pitch diameter:
(Number of teeth)/(Diametral Pitch) = (50 teeth)/20 = 2.5 inches
Pitch radius = 2.5/2 = 1.25 inches
A summary of spur gear properties here:
http://shopswarf.orconhosting.net.nz/spur.html