Been a long time since I did one of these measurements but there are only two variables to play with: the fundamental frequency and the level. The standard frequency I always used was 1kHz. As for level ... if I remember correctly, I think I put the amp into clip and then backed it off till the THD stopped falling. This also is probably the best point for THD+N.

That said I'm sure some played around with both and then only give you the best THD number.

One more thing ... peoples sensitivity to THD varies with frequency ...