Tensioning is kind-of an art form.
My advice is make it as tight as reasonably/easily possible with just your fingers, without using tools to pry on it, and when you pinch the belt around the motor pulley with your fingers it should squeeze about 5mm or so.
There are also tensioning methods that involve plucking the belt with a hex key and using a guitar tuner to tune the belt pitch, but I don’t recommend that. If you do that, maybe 150Hz.
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Hold the motor can and try to force the wheel to spin the same direction it’d be spinning during braking.