A little better guess can be achieved by realising that the total indicated horsepower is the number that should (all things being equal) vary with density altitude. Then you have to take off a figure for mechanical losses, pumping losses and (maybe) supercharger power requirement. That's gonna be something like a fixed number. Say you have a 100hp non-supercharged engine. It has an IHP of, say, 120HP. 100hp at the prop and 20 taken up by losses. At altitude corresponding to a loss of 20% of density you lose 20% of 120hp that leaves 96hp indicated. But you have to take off that fixed 20hp in losses. So you end up with 76hp. You lost 24% of your power at the prop. Different engines will have different figures, of course. I've seen an estimate of 13.2% of IHP in losses for a typical non-supercharged engine. Seems about right, but only typical not specific. With a supercharger, mechanical or turbo, things get more complicated.