P.S. I have a Master's in Aerospace and have worked in the industry for
many years. I will admit most of my schooling and experience was with
jets and rockets -- not pistons and props. But I do recall the equations
and techniques to calculate engine horsepower required for various flight
modes of a prop plane was VERY complex.

Yeah, those damn eggbeaters hanging out the front make it all pretty
complicated. I most certainly DON'T have a Master's in Aerospace. I find it
slightly comforting that a guy that does says it's complex.

Thanks for taking the time to answer.

I am CERTAIN equating thrust horsepower (thrust times velocity) to brake
horse power (torque time RPM) is wrong. Anyone have an aircraft
performance chart to look at the IAS for 75% power at sea level and at
altitude?? I am not going to say it will be exact, but I think it will be
close.

Do you mean working back from TAS to get an IAS?

I looked up a Navajo information manual. There's a chart True Airspeed vs
Density Altitude. I chose the line for 260 BHP which is around 75% of the
350 BHP engines.

At sea level the TAS is shown as around 207 MPH (have to interpolate, it's a
grid that goes up in 10s). That is obviously the IAS as well.

At 20,000, the TAS is close to 250 MPH. The inferred IAS is 184.

Any thoughts?