Madison,
We've checked our calculator against
SDP-SI multiple times, it is always within in .0001 before adding tolerance. When done properly, the belt should be tight enough that there is some difficultly in assembly. In all of testing where both pulleys are located on the same plate, the calculator yields these results (9.5305")
It's difficult to see exactly in the pictures, but it looks like the pulleys are not being located on the same part, but across multiple plates. This can cause issue's depending on your manufacturing tolerances. On 1323 this year we had two belts that were located across three plates, and we did notice the belt was looser than normal. However, it is still tight enough not to ratchet. I would recommend to first try to measure the actual center distance on the robot. You could do this with a big caliper, or if you have the ability, you could make one plate with the exact center distance and compare it to the robot.
As far as the belt breaking, the loading on this belt does seem rather high for a 9mm belt. Your reduction has a max torque of 932in/lb, whereas an average 2 CIM drivetrain in low gear has about 650in/lb max torque. I would definitely recommend switching to a 15mm belt if you can. Some more info about how/when the belt broke would be helpful here.
We've been using HTD belts and the belt calculator on 1323 for two years now and have been very happy with them. The only time we have had a belt failure was when we cut a 9mm belt in half and then used screws to join it back together. Since then we avoid cutting belts in half, and try use 15mm wherever we know its going to be a high load.
Hopefully this helps you out, let us know if you find anything else out. As always you can contact us a
support@wcproducts.net or
prosupport@vex.com.
-Alec