The best thing I have found for doing this is a circular sliderule sold
by Far West Aviation called the Power Wheel.
This thing is pretty neat. Read about it he
http://www.farwestaviation.com/about.html

It condenses all the hard to read data from the graphs in the Lyc
engine manual onto a circular scale
that will give you the percent HP, including altitude, temperature,
RPM, and M.P. It also gives a leaning
recommendation base on their experience with detonation. They give a
very extensive seminar of the
topic. My only gripe is that some of the printing on my Power Wheel is
a little muddy. Maybe I just got
a less than perfect one. They have a 100% satisfaction guarantee so
maybe I will call them up and see if they
are all printed that way.