Not sure what your asking, but if it's to find the fastest rpm that you can drive the motor then the following may help. If it's for sizing the psu then each motor usually has a maximum power rating, depending on how you drive them then the voltage is a bit of a red herring as most motor drivers run the current through the motor windings in a controlled manner, usually at a much higher voltage that the motor specs dictate. ( Gives a much faster motor response ).
Unfortunately it's not the holding torque that you need, for each stepper motor that is manufactured there is usually a speed/torque curve, what you need to do is calculate the cutting forces ( push ) that you require, convert this into torque needed to drive it ( differs depending on drive mechanism whether ballscrew, acme leadscrew or toothed belt/gear train - all have different efficiencies and losses ), once you know the minimum torque you can get away with that will drive the system then you go to your speed/torque curve and that will give you the maximum rpm that you cannot go above. This is a very simplistic approach since you really need to figure in the masses involved that you want to accelerate and decelerate.
Stepper motors behave differently to most other motors the maximum torque is developed when the motor is stationary and falls off with increasing speed. I guess that's why they are sold by 'holding torque'.
Hope this helps.