Get in the books
Get into your engine owner's manual, there's power curves.
You'll want to be below 75% power when cruising around so you can get the most life out of your engine.
Using your manual you can determine fuel flow at the different altitudes and power settings.
For my angle valve 200 h.p. Lycoming IO-360 the charts are based on running 150* ROP.
Remember, the charts don't show fuel flow for LOP, so you'll have to baseline your own fuel burns for LOP operations.
I put a little chart on my checklist that shows fuel burn, power settings and % power based on the the Lycoming manual. If I pull the red knob significantly more aggressively, to 30* LOP, my fuel flow goes way down at the cost of about 8 knots. My CHTs come down and even out as well.
My wife, who occasionally flies my RV-7, asked for a cheater chart for power settings when she's cruising around the local area and when going cross country.
3000 feet - 20 inches - 2400 RPM - 8.2 gph - 55%
6000 feet - 20 inches - 2400 RPM - 7.6 gph - 57%
6000 feet - WOT - 2400 RPM - 9.8 gph - 71%
8000 feet - WOT - 2450 RPM - 9.0 gph - 65%
8000 feet - 20 inches - 2350 RPM - 8.0 gph - 55%
These are Lycoming book numbers for the IO-360 angle valve engine (200 hp).
Note that lowering the prop RPM to something like 2350 or 2300, depending on your recommended limits for your engine and prop combo, can reduce fuel consumption because it reduces % power produced. It is worth testing to see what changing prop RPM does to your airspeed.
When sight seeing locally or just enjoying a beautiful day at 3000 feet MSL and not going anywhere, setting 20 inches of manifold pressure with 2350 RPM is relatively quiet, keeps the engine very happy and is plenty fast at around 130 knots indicated, plus or minus a few knots. There's no prop RPM limits with my Hartzell two-blade metal propeller and the angle valve IO-360, so occasionally I'll dial it down to 2000 RPM while just flying in the local area.
When going anywhere the most efficient power setting seems to be WOT at 8,500 feet and 30* LOP. If you want to go faster run 150* ROP and gain 10-12 knots and burn 2 gph more fuel.
Get in your plane and go experiment. I love flying with a purpose and a great excuse to go fly is testing things to see what works well for my own flying style.
Does that help?