Power -or thrust --
a bit tricky
The engine has less air to breath --but the prop can churn the air more easily--
So- same prop at any given rpm will produce less thrust than at sea level
How much?
Again, tricky
The only definitive test is a spring scale
turn a given prop at a given rpm at sea level- record - then turn same prop at same rpm at altitude -
compare
Let's say that the test shows 12% difference - ( this depends on corrected altitude density)
Now lets add power till we get same thrust .
How do we do this?
Remember - we are reading prop results -not engine setups
OK let's say-we can simply get more power out of the engine-until the thrust is equal.
Now we have the power (thrust the same)
Will the plane fly the same ?
No.
It will fly faster with same thrust because air has less resistance.
The slow speed performance will still be inferior because it takes more speed to develop same lift.
Actually, being lighter -will make the most difference in how the plane performs.
So --we add pipes to engines and carve off all the weight possible from our airframes.
Some highly touted combos -at sea level -are simply duds at our altitude.
So what is the "rule of thumb"?
I don't have one - just add all the power you can without adding weight-and if possible loose all the weight possible
So far I not seen one too light with too much power.