From the Gecko G251 manual -

A very accurate calculation of power supply voltage is to find your motor’s inductance, and put it into the following equation.

32 * (√mH inductance) = Power Supply Voltage

If your motor has 2mH of inductance, the equation would look as follows.
32 * (
√2) = 45.12V


So, using the 6.5mH of your motor would give an optimum voltage of 81.58V.