wrote:
I think most people are recommending 6000' to 8000', because it gives
75% power. is this just "experience" or "rule of thumb that I learned
somewhere" ? in other words, why is 6000-8000' where the air
resistance vs. power curves cross? or, why is 4000' not better? there
is more power. why is 10000' not better? there is less air density.

Keep in mind that this is "density" altitude, not "pressure" altitude.
To understand why, you have to remember what the throttle controls.