Very simple. Run your axis from a 24VDC power supply. Keep increasing your speed until the axis becomes unreliable. Back-off to the last reliable speed and mark it down as your "what you got" speed.
Divide your "what you wanted" speed by the "what you got" speed and multiply it by 24VDC. The result will be the optimal power supply voltage.
Example: Say you got 50 IPM at 24V and you wanted 100 IPM. 100 divided by 50 is 2. 2 times 24V is 48V it is your optimum supply voltage.