I usually fly 500nm straight legs, the only turn is on the traffic pattern, so I always choose to cruise beetween 10 to 12,000ft.
For a long range cruise, at 150KTAS at 12,000ft, I just leave the throttle at firewall, rpm at 2100 and mixture at peak EGT.
If you are among those that still think that a lean mixture is harmfull to your engine, you need urgently to read John Deakin's columns at AVweb.com.
I'm just waiting the next overhaul to install my GAMIjectors so I can go even leaner, may be 25 to 50 F lean of peak.
Thank god the new engines and some new POH are coming with a recomendation for lean of peak operation as the best economy setting (like the Cirrus).