If it helps, I did a video showing how I tension my belts, you can see it at X-Carve Maintenance/Troubleshooting Videos - Add Your Own!.
My belts are tensioned to where it takes about 3.5 pounds to pull the belt up at mid-span, and it seems to work just right for me. I had them up to 6 pounds at one point, and broke 3 belts within a couple weeks.