That's the part I don't understand. Don't these stepper motors work by having multiple coils interacting with multiple rotating magnets, arranged so that the rotor tries to drop into alignment with the energized coils? The torque generated surely depends on the misalignment between the rotor and the coils, and the current through the coils, not just on the current. I don't see how you could know what the actual delivered torque is just by looking at the currents and voltages across the coils. If there was no load on the rotor then you could put as much current through the coil as you wanted (short of actually burning it out) and the nett output torque would still be zero.