this is in fact tricky: The 300 Step 3 phase motor has 50 electrical rotations per mechanical rotation. The TMC389 splits each electrical rotation in 1024, 512, ..., 8, 4 "microsteps". But, transferred to the three-phase motor, an electrical roatation is 6 fullsteps. Thus, when you set 256 microstep resolution, the resulting real microsteps is 2/3 of this value, e.g. 171 microsteps. At electrical fullstep it is 2/3 microsteps. This means, that with MRES=8, you get 1.5 steps of the motor - this is not true fullstepping. So, it is much better to work with higher resolutions.