Nope, that doesn't work. The amp will only be so efficient, and that cannot be calculated by fuse value. If you want to test the true power, you'll need a voltmeter on hand, and a resistor for an impedance load. Use the instructions here: http://www.bcae1.com/measpwr.htm
Well that is what you said in one of your post(Archive through June 30;12" volfenhag;7 post)I just thought that it was right since you posted it.
When I said that, it was mainly to prove that certain cheap amps are in no way capable of producing the peak powers they claim, but it also doesn't take transformers into account. It balances out when efficiency is factored in. Anyway, the above is the proper way to measure RMS.