its late so bare with me
just a few quick notes
-manufacturer specs and actual outputs are different. It seems the method you went about used calculation sbased on what the amp was supposed to do. Now thats fine but most times the amp is off so calculating it this method would be off a bit
-Most decks will hit volumes near 95-99% or even 100% volume sometimes without clipping on the low level output. I have seen it on scopes. Some decks will not produce a clipped signal at very high volume on the pre amp side, but on the speaker level side the output is very distorted
-Best method for setting gain is still using an 0-scope if you really want to take a good reading, and it has to be with a load on the amp, dont disconnect the speaker load, just double up with the o-scope probes
-Next best method is using a True RMS meter and turning up the gain untill the AC voltage stops increasing then turn it back down a notch, this will indicate on your MM that your starting to clip, without actually being able to see the wave
-The best method I still find if you don thave a scope is an ear, just train it well and you will be happy with the output. Some peopl ehave great ears and can pick up everything, and some can't hear worth a dime.
Not a bad article, good to see this out there to help newbies. Anything like this is always a help.