OK lets make this simple.
On the Rich side of peak EGT, the % power is determined by MASS AIR FLOW. In other words you have more fuel than you need and hence the volume or mass of air being consumed detremines the power.
Simply, less swept volume (RPM) and less MP (O2 per said volume) determines HP.
Sure the cam design etc affects this, but lets say 20% less MP, and 10% less RPM, is 0.8x0.9 = 0.72 this 72% power. Now this si not always accurate but it may be close enough.
On the lean side of peak EGT, in a normally aspirated 8.5:1 engine take fuel flow in US GPH x 14.9 = HP.
How is that?
Another thing you could learn if you did an APS course
PS, Out of the three levers available, Throttle/RPM/Mixture, which is the most effective power lever?