Philip,
A lot depends on the number of teeth you need on the sprocket. Small numbers between 16 and 20 require a different calculation to higher numbers, from about 35 and up there is a small difference from gear tooth calculation. The system is orthagonal, in that the drive is not a constant speed due to the straight line links from one roller to the next. This induces a torsional oscillation in the drive which is why motorcycle drives incorporate a 'cush drive' system.
I have some sprocket cutters which are for 0.25" roller x 3/8" pitch, and one which is for a 0.335" roller x either short 1/2" or long 1" pitch! I see Clive has posted a link for a calculator which has saved me digging out my calculations for the sprockets I cut – generally they were 18/19 tooth gearbox and 43 – 53 for the rear wheel on my BSA racer (see avatar!!)
