Uh, make sure you have a voltmeter that can indicate true RMS up to 400 Hz?

Ohms law still works. However, the resistance of the wire is different at 60 cycles than it is at 400 cycles. Google has the answer.

Well, first you have to rectify it to DC, bleed about 5% to ground, roll some dice and throw out the remainder.

Or you could just use the same math you use at 60 Hz, provided you take into account the change of impedance for any inductors or capacitors due to the frequency of the power system (or signal, or transient, or burst).