I think there is a lot of leeway in the wattage rating of the resistor. The rating given in the manual has to cover a lot of variables. For example the VFD may be connected to a conveyer belt (or a lathe that is turning a heavy work piece) that has a lot of inertia that has to be dissipated. A milling machine has vey little inertia by comparison. All you are slowing is the motor and whatever cutter and holder is in the spindle. Not much mass. The short time it takes for the motor to come to a complete stop is not really enough to put much heat into the resistor.