As always, I trust Misumi to be a very good source for information:
http://uk.misumi-ec.com/vona2/detail/11 ... 96%3A%3Aag
So, the important decisive dimension is the length and position of the core wire.
The length is ( n * 2 ) with n being the number of teeth, and 2 being the "GT2" 2mm pitch
This means that if I were to wrap a 60T belt around that 60T pulley, I would have a 120mm belt circumference, which results in a belt ( = core wire) diameter of 38.2 mm (120mm / pi)
Doing the same for the 20T Pulley, I get 40mm circumference and a core wire diameter of 12.7mm
Now, I just add tangents to the two circles, and measure their length:
Then select the nearest valid belt length, which would be 154mm (77 teeth) in my case, and adjust the axis spacing until I get the correct value:
Does that sound ok, or am I missing something?
Now, I have a very nice CAD program that lets me do this comfortably. But is there a general way to mathematically describe those tangents? It would be really nice to have an Excel spreadsheet where you can enter two diameters and a spacing and get the belt length...
/edit:
About belt tension:
Mounting the motor (smaller pulley) with slotted holes should solve that problem without having to do a lot of calculations. I actually have slotted holes in the parts that I already had lasercut. I just noticed that there's an error in my SpaceClaim drawing where the slotted holes are actually round holes there. Gonna fix that error.