Don't lean too much. It does not seems so much necessary in game.
Before entering a fight, you can set it to full rich or if you are really high full rich minus half an inch
At cruise I use the "L" letters of the "Lean" indication as a mark with the lever of the mixture. Usually, both are aligned together at 20k Ft. With both boost and rpm at 12o'clock (rpm slightly less) I use to get 200mph cruise speed that way. The Temp stabilize itself around 3 o'clock. But I can lean even more but with a reduced boost to max out my range (over modelled IMOHO - I am sure I can go right to Berlin and be back with a case of schnaps for the poor German fellow that had to bail out and convert to green tea that way)
But I hve learn that it all depends of the plane you get and the way you warm your engine before taking off. I found that it both impact the way an engine temp raise. In the worst scenario you can then barely use full throttle before the temp start to coast raise to a critical value. Very disturbing in a fight.
So if I am committed in a fight while flying at cruise speed the scenario is the following :
Nose offset
Mix on rich
Temp check
Pitch lower
Throttle frwd
Boost check
Increase pitch
When Speed > 220/240 Rad on med
and then what you alrdy now
Gunsight On (the lamp of the gunsight were prone to fail hence the sight was illuminated only when necessary - not modelled)
Check 6!
Nose on target
Check 6!
Rock and Roll
I hope it helps.
~S