On my old Singer treadle machine I put the belt into place and pulled tight, marked it with a pencil, removed it from the large wheel at the bottom and cut 1/8" out, put the belt back into place, tested it, removed the belt again, cut another 1/8" out, put it back together again and tested it. That is how I finally got mine tight enough by myself!
I use an old Singer hand-crank machine now and I love it! It's fun and makes perfect seams!
Vanessa in Oklahoma