I think I would punt and either:
- determine it empirically by driving the motor n steps and measuring the angle before and after, or
- use servos instead of stepper motors. servos are already designed to go to angle X and stay there.
OTOH, I don't see a huge math problem here. You just want to determine degrees of rotation?
So, imagine a 180 step stepper motor. So each step is 2 degrees. If you want 0.5 degree per step, then you'll need a 1:4 drive ratio.
Here are some stepper motors. Note these are all 1.8 degree steps. Or, you could by a 20-step per turn motor, then each step is 360/20 degrees -- for that, 1:20 drive ratio would get you 1 degree per step.
Precision Stepper Motors | Hansen Motors -- 7.5 and 15 degree steppers. You can also get steppers with attached reduction gearboxes.
This is just multiplication and division, to determine how many degrees you get, based on your drive ratio.