You, in fact, want to adj. the top rpm's with the blade rotating it's the only way to get an accurate reading.
The blades being engaged is not considered a load. It is a parasitic load, and should be accounted for when setting the top no load speed, which, by the way can be set to any rpm you want on your personal mower. I like to keep my unit at 3500 or so.
Blade tip speed/ rpm charts based on blade length/ cutting dia. are online.