Any PWM based servo drive applies FULL buss voltage to the armature.
The voltage the motor "sees" is based upon the ON duty cycle of the drive at any point in time.
Assuming the buss voltage is 100vdc and you give a 25% of max speed command, the averaged voltage will be 25vdc and the motor should approximate 25% of the speed the motor would run at if the 100vdc were applied to it.
However, the motor terminal voltage may not measure 25% with a vom due to the averaging algorythm that the meter uses. Give a 50% duty cycle, and some simple math and you have your "formula".
In reality, the motor "sees" the full 100vdc while the amp is in the ON mode BUT, BUT it only really "feels" this 100% voltage 25% of the time with the example.