Coming back to this - the thread was started with malice aforethought; I did the trip I had in mind this weekend - 8500 out, 9500 back. Fairly high OAT generally - 43C on ground / +20 cruise out, and 30/15 back.
Both ways the chart showed me to be above full throttle height, so I naievely expected to push the black lever fully forward and lean for max rpm.
Interestingly I had to keep the throttle some way back to keep the revs under the red line (cruise was at about 2650rpm, PA28-161). What did I miss?
POH showed 75% at full throttle/2700rpm. Was very thermic so didn't risk running at 2700 due to lack of wobble room...