Here is what I would suggest. Fly at the field elevation in level
flight (obviously somewhere where the ground is lower). Lean for
maximum RPM. At that altitude (7000+'), you don't have to worry about
overleaning. Just lean to peak rpm. Now go and land at THAT setting.
Now leave it at THAT setting and go do a runup AT THAT SETTING. Turn
the mixture knob some both ways to see what the effect on peak rpm is
at your runup rpm. NOW USE THAT mixture/rpm behavior to lean. It will
probably be right at peak, but if it slightly below then lean to that.
Hope this makes sense. Basically what I am advocating is leaning to
peak in the air at full throttle, then go on the ground and learn how
to recognize THAT mixture setting based on runup rpm behavior. Good
luck!

If all else fails, just runup at full rpm and lean to peak rpm. That
will work also. (A bit risky with the prop blast at full rpm, but if
you watch where you do it and are careful can be done).