Matt,

My question: Is the engine
developing less than 75% power during the climb; and is this why leaning in
the climb is OK?

Yup. That's where the 3000 feet come from. Because of the lower air density up
there, your airplane will develop less than 75 percent rated power up there.
Two big caveats:

1. We're talking density altitude here. So you might well be leaned during the
roll down the runway at 100 ft MSL elevation, if the temperature is high
enough. Not rare at all.

2. It might make sense to lean even earlier when you want to maintain max
power. One advice given is: Keep the EGT where it was when it stabilized after
took off at full power. If you don't lean, EGT will decrease slowly with
altitude.



--
Thomas Borchert (EDDH)