I use the Gates smartphone app, insert two 1/4" endmills under the belt (the SHAFT part …1/4" dowel pins would be safer) 280mm apart (for my small SO3), and pre-tighten the belts by feel, such that I still have a quarter turn or so left before the bolt is fully tight against the plate, then twang the belt a couple of times to get an average frequency readout, tighten/untighten the bolt as required until I get ~120Hz (on my SO3, larger machines require different values as Liam mentions).
…1/4" dowel pins would be safer) 280mm apart (for my small SO3), and pre-tighten the belts by feel, such that I still have a quarter turn or so left before the bolt is fully tight against the plate, then twang the belt a couple of times to get an average frequency readout, tighten/untighten the bolt as required until I get ~120Hz (on my SO3, larger machines require different values as Liam mentions).