As a first approximation, it should be the same for any normally
aspirated engine (and yes it would be based on DENSITY altitude). The
engine develops 100% HP only at sealevel. As the altitude goes up what
changes? The density of the air decreases and in response the fuel
delivered decreases (from both the carb delivering less fuel and the
pilot leaning the mixture). This is the same for all the normally
aspirated engines.

Now at somewhere around 65-70% power it becomes impossible to get
excess EGTs and CHTs due to overleaning the engine. So run the engine
at that altitude and lean for max rpm and note the EGT. This is
guaranteed to be a safe EGT! Now use this EGT for leaning at ALL
altitudes. Thus you have found 100 or so degrees rich of peak without
ever having to run the engine at peak (which is too hot and hazardous
in itself).

Doug wrote:
Anyone tell me at what altitude I get 75% and 65% power respctively
(with full throttle, normally aspirated).