Isn’t it easier? For DRV8825, supplied max current is Vref * 2 (from the datasheet).
So for coils rated at 1.2A, max Vref is 0.6V , or 1.2V for 2 motors in parallel. I wouldn’t go that high — about 70% of the values should be enough.
You don’t need to do any complicated calculations.
Resistance of coil = 3 ohms. This doesn’t change (outside of heat effects)
Max current we set at the driver: 1.2A (ideally around 70% of this)
==> V = 3 * 1.2 = 3.6 [which is correct; the rated coil voltage]
No need to worry about 12V anywhere.