NOTE: when planning to change from printed to aluminium idler and motor mounts add several cm extra length to your belts as the idler mounts will move the idlers further outside then the printed ones.
Using a guitar tuner (or guitar tuner app on your mobile) can assist with belt tensioning. Belts have been measured to usually be around 60hz on the long return.
5m of belt is enough to do both belts on either printer. The 300ZL is roughly 2.2m of belt per side.