Generally timing belt is complicated device and many thing depents on its internal construction and materials (thay may be spoiled when bound in wrong direction, or when cut, and when overtighten of course, etc.). There is also physics and math behind based on [Mersenne's laws][2].

Some vendors provide calculators (online or as phone apps), which can calculate tension (force in Netwtons or lbs) or the frequency (Hz). Therefore often the advice is to tension belt until some (bass) sound is present - and professionals would tune belts with a sound tuner. There are also hints that belts should be possible to connect with fingers with slight or sgnificant pressure (so not consistent). There is also visual guideline: when you slowly move the carriage with hand, the belt should remain straight. (Slowly, because belt is elastic and may behave different when moving carriage stronger and faster against friction of pulley.)

I would suggest to read [this article on 3dprintingspot.com][1] for many practical suggestions.

  [1]: https://www.3dprintingspot.com/post/3d-printer-belt-tension-everything-that-you-need-to-know
  [2]: https://en.wikipedia.org/wiki/Mersenne's_laws