Hi,
there is no magic number.

Microstepping came about because astronomers wanted increased resolution. As it turns out beyond 1/2 stepping that does not work, nice to imagine it does but in practice the best
resolution you can get from a hybrid two phase stepper is 400 steps/revolution. What astronomers did find however that increased microstepping made the motion smoother and that suited their purpose
trying to drive telescopes with the least possible vibration.

The higher the resolution the smoother the motion is, but you tend to lose torque.
That is simplification. Certainly you get smoother motion with microstepping but not actually reduced torque. The issue is the torque between adjacent steps, and that does reduce.

Lets say you have a motor of 1Nm per full step ie 200 step/rev. Most motors will offer the same torque at half steps, so 1Nm @400 step/rev. However at quarter stepping 800 step/rev the torque between any two
steps is halved, ie 0.5Nm. It gets worse at higher microstepping, lets say 16th stepping or 3200 steps/rev the torque between any two adjacent steps is 0.0625Nm.

In this situation you might call for the stepper to rotate one revolution say. A step will be issued but because the torque is so low it will not move until say six or eight steps have been issued and NOW there is sufficient torque
for the motor to move, and move it does.

At about eight micro steps per full step (1600 step/rev) you have got the best increase in resolution that you are going to get and have gotten about as much smoothness gain as you can WITHOUT having to
signal the drives at light speed.

That is my recommendation set the micro stepping anywhere between 1000 step/rev to about 3200 step/rev....and leave it there. You've got about as much benefit as you can without getting into
trouble with ultra high speed signalling.

Then you just have to set the Steps/Unit value to suit the mechanics of the machine.

Craig