My aerial photo flying requires me to not be screwing around "getting gas" when I'm out trying to do aerial photography. I want to stretch what gas I have as far as I can. Ben Visser (GA NEWS) the other day said 65% power will use less gas than 70% power. I know he is right. But just how do I figure out what EXACTLY is my percentage of power via the RPMs that I am turning? What math do I use? How do I do it? Does altitude affect it at all? Is there a point where your RPM setting is using less gas alright but at a cost of too much less speed? Maximum range comes down to MPG I would think? Again, what is 65% power in a Lycoming whose max RPM is 2700? And how is that RPM computed?