"Don't do it at 100%, wait till you've settled into 65-75% RPM"
Forgive me for being pedantic, but that's not quite correct. You should cruise at or below 65-75%
POWER - this is not the same as 65-75%
RPM, and it's not the same as 65-75%
THROTTLE. If you cruise at 65% RPM, I'd be surprised if you maintain straight and level!!
Example for my own aircraft (a 1959 C172)...
I generally cruise at a gentle 2300 RPM. This is 85% of the rated RPM (2700). At 2500 feet, this is only about 57% power. At 5000 feet, it's now only 54% power. At 7500 feet, it's 51% power. But note that all these figures are for the same RPM.
I'm always below 65% power in the cruise, so therefore I always lean at all altitudes - even when pottering along under the Gatwick CTA at 1500 feet. This gives me a fuel-burn of about 25 l/hr or thereabouts. I don't bother leaning in the circuit though - there simply isn't enough time.
You need to take a look at the POH for your aircraft, which should contain a table telling you percentage power at various combinations of RPM and altitude. Remember also that "percentage power" is a percentage of the
maximum rated power of the engine - i.e. the power achievable at sea level. Even at 2500 feet you'll probably not get more than 85-90% power even at full throttle.
Obviously all of this applies to non-turbocharged, fixed-pitch aircraft.