Hi, Hugo
If your not using Mach3R2.0.050 latest, you should be, it has much better Rotational views.
There are 3 places to setup the rot axise's, General Logic also "Rotational" Rt side center lower, 2 options,
Tool-Path config., Rotstional, center top, 2 more options there.
Then Motor-Tuning, You need to take the number of step's Required to make 1 full turn of your rot. axise's and divide it by 360 to get
steps per Deg., Enter that value in your Steps Per window, it puts you in Deg. Mode.
An example my sherline rot table is 10 micro step it takes 144072 steps 1 turn dev. by 360 = 400.2 steps per deg, vel 2500 acel. 500.
This should get your Rot axies in deg., Not sure that a axis acts as it should compaired to b,c give it a good test.
Hope not too much or too litle Hear, Chipl