I'm using a HTD5 15mm belt to join a big stepper with a shaft, I've used belt / pulley calcs available here:

SDP/SI

I've left 15mm of space on both sides of the stepper, to pull it back a bit and have some tension on the belt. Is that normal practice, or should the pulleys be at precisely the distance shown by the calculator? I'm asking as I've pulled the belt a bit to have less slack on the belt, but after some running, there's tiny bits of rubber under the belt, as if something is rubbing or stressed too far. The belt is not pulled too hard, I'd say it's medium tension, I can still pull it with my finger and it will give slightly. Just wondering if tiny rubbery residue under the belt is normal or not.

Regards,
dsc.