When you have checked "A-axis as angular" and configured your steps per unit to achieve 360 units = one complete rotation you may choose a couple other options. Namely the "Rot 360 rollover" and the "Ang Short Rot on G0". Play with these options and put G0 and G1 moves in the mdi line to see what happens.
As Greg asks, how do you see Mach threats the A-axis in mm and not in degrees?