Most POH have MAP/RPM combinations for % power. You want a 75% power climb? It's probably there for you. You want 85%? Probly find that too. Since these figures come from test cell work by the engine manufacturer, you'd be pretty hard pressed to convince anyone who is semi intelligent that "word of mouth", or the "know all down the road" knows better. These work just fine. That's why they are there. Comparing car and aircraft engines together is a completely fallacious argument. An aircraft engine spends 90 % of it's life producing 65 % or more of it's rated power. Your average car engine spends 90 % of it's life producing 15% of it's power. Two completely different objectives.