[lost2a6]

It's EASY to find out how hard you're running it! Just install a vacuum gage and see how much vacuum you're running at your typical cruise rpm and load. If you're at or above 7" Hg at cruise, you're ok. Mercruiser used to recommend 5"Hg minimum as a Max Continuous Power setting.
Less than 5" and you can expect reduced exhaust valve life. The higher your cruise vacuum level is, the cooler the exhaust valves will run.

A vacuum gage is also a very sensitive diagnostic tool that can pick up subtle performance changes that you otherwise might not notice until it gets a lot worse. I'm amazed that more folks with inboards don't use them. All the engineers I worked with at Pratt, who developed new engines for a living, and ran inboards, had installed vacuum gages and used them to monitor engine performance/health.

Not to hi jack this trend, just curious about vacuum readings on an engine that has some valve overlap. My 502 barley has 7" of vacuum in gear idling. I've never checked it on plane at cruise, however I could only imagine that it would continue to drop from its idle vacuum as I give it throttle. We might have to go on a certain percent of an engine's max vacuum rather than a fixed number such as 7". Every engine model is going to have a different vacuum reading based on cam design, engine displacement, ignition timing, etc.