This is really helpful for developing belt drive: http://www.maxtorque.com/calctool/ce...nterstartx.asp

As long as you know the ratio you want and the distance between wheels, it'll tell you all the possible options. As long as the number is with .01" of your goal, you'll be fine (though it's better to be .01 under than .01 over). For a for-sure fit effect, couple this with a tensioning system, and you'll get a belt drive that'll never fail.