Actual value for timingbelt tesion.....

[Voicecoil]

So I went and googled, and eventually found a data sheet with some maths in it and the recommended figures for pre-tension are suprisingly high, something like 1/3 to 1/2 of the full running tension for a 2 pulley system, more if you have multiple pulleys. This means that for a 4Nm Nema24 stepper and 27 tooth AT5 pulleys (which are quite big) you're looking in the region of 60...90N, which is more of a radial force than some steppers of that size will be happy with: and with smaller pulleys things would be worse.

[Robin Hewitt]

With CNC precision I don't bother with tension, I simply put the wheels to the correct separation and it just works. If using a belt for linear positioning then you should know what tension you require and make sure you have enough teeth engaged to carry it.