For the stretched belt, you might be thinking of 1425. First of all, we didn’t calculate much, we ordered a 20in bearing and a large belt that just happened to fit over the bearing with a lot of clamps and manpower. To drive the turret, instead of using another belt around the outside, we used a custom made 54T delrin pulley with no flange. We had said pulley slide so we could tighten the meshing to account for any errors in calculations of the pulley size.
Towards champs as we were trying to speed up the turret, we found issues with the pulley skipping teeth during hard acceleration or deceleration. If we had to do a turret again, we would now use a bearing sandwhich or large X-contact bearing driven by a large custom made sprocket or gear. Belts are bad
We too found that two belts how you described, do not mesh properly which why we went this route.
Feel free to ask any more questions.
-Daniel Kassab Driver 1425