Hello everyone I have a tos fnk25 vertical milling machine and wish to convert it too cnc .I am a bit stumped as regard the size of stepper motors, I require, can anyone help?

here are some figures i worked out
require 640oz/ins when load applied to x axis
would like to have 2 to 1 gearing
max 833 steps/second with gearing
according to gecko site
this relates to 163 watts of power need to move the axis
833 x 640 divided by a factor of 4500 plus 40 percent
have i work this out right?
how do i now pick the most efficent motor?
if i go oversize on the stpeper motor .can i reduce the voltage to compensate thus reducing the power?

all the best Derk on the Isle of Wight (uk)