Motor drive belt tightness

I bought a retrofitted Bridgeport clone mill that has size 42 stepper
motors on it. The controller was blown up and in the process of trying
to debug it the previous owner had loosened up all the belts on the
machine.

It has 5/8" wide Gates type cogged belts on X and Y and a 3/8" wide
Gates belt on the Z axis.

Is there a "rule of thumb" for how tight these belts should be?

Thanks,

Paul T.