Shouldn't be too difficult to calculate.... First you'll need to know the rotational inertia of the spindle.....

Not only do I not know that, but I'm not even sure what units it should be measured in!

The only things I know about the motor are:

Voltage: 12v-36V (I'm running at 36v with a PWM controller)

12V Load Current: 1.7A (starting current 2.2A)

Stall Current: 4.7A

Load Power: 20.4W

Speed: max is16,300 rev/min at 36V with 100%PWM

Looking around, sites seem to suggest anything between about 0.5-10 ohms, and 10-50 watts. I'm not sure how critical such values are. The resistor would be wired into the crash stop switch, so the motor would either be connected to power, or the shorting resistor, with no possibility of any other linkages...