Well, actually you don't.
You set the valves with the piston a fraction of an inch down the cylinder past TDC. That is what the manual says.
Nobody takes the head off one of these engines to set the valves if they can get away with it. The only way to get an idea of where the piston is is to remove the blower cover, rotate the crankshaft until the piston comes up on comression, and then rotate it until the back edge of the magnet is just about past the magneto. At this point the piston should be starting down the cylinder.
Then adjust the valves at this position.
I don't see what rotating the engine backwards will do for you. I don't think the compression release retracts until it reaches about 300 rpm.