Good question. The best answer I can come up with is about 3% per 1000 ft in "standard atmospheric conditions". That fits almost exactly with the aircraft rule of thumb that you can only get 75% power at 8000 ft.
The other major factor will be mixture control. That 3% per 1000 ft will be assuming you can keep the mixture optimized by reducing the amount of fuel flow to match the reduced air density. If you can't do that, then you'll be running richer than optimal, which will reduce power even more.
Question: RV 4 equipped with Rotax-914 engine and variable pitch propeller.
Problem observed during flight above 7000 feet , vertical speed become 0, rpm over shoot for 2,3 second up 6300 when turbo engaged for gaining height.i did effort for Turbo but not gaining height. Feels like under power. But take off was normal. Air Box target pressure normal. All other engine parameters also normal.
So come back to pavilion.
Suggest possible causes please.