pulley center distance vs. belt tension

[m_c]

Puller centre distance isn't an exact science.
The most accurate way to set tension is by harmonics aka you ping the belt and measure the frequency at which it resonates.

For something non-critical, I'd probably verge on the side of lose rather than tight. I wouldn't worry too much about little bits of rubber during initial use, however I'd keep an eye just to make sure the belts not wearing unevenly.