Generally, a timing belt is a complicated device and many things depend on its internal construction and materials (it may be damaged when bound in wrong direction, or when cut, and when overtighten of course, etc.). There is also physics and math applicable, based on Mersenne's laws.
Some vendors provide calculators (online or as phone apps), which can calculate tension (force in Newtons or lbs) or the frequency (Hz). Therefore often the advice is to tension the 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 significant 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 for many practical suggestions.