Some people have said that you can take the manifold pressure in inches, and add the rpm in hundreds. If the total is 48, that is 75% power. If it is 45, that is 65% power. If it is 42, that is 55% power. This would be very wonderful, if it worked.
But, things are not so simple in the real world. If you dig into the Lycoming power charts, you find that for a given manifold pressure and rpm, the power produced goes up with altitude. The simple formula above doesn't take altitude into account.
I fired a few numbers into my IO-360 power spreadsheet. The rule of 48 isn't too far off the mark at altitude (i.e. near the highest altitude at which you can reasonably expect to get a given percent power). It is quite inaccurate at lower altitudes. For example, using 48 (hundreds value of RPM, plus MP = 48), I get percent powers that range from 68% to 74%. 45 gives me between 54% and 65%, and 42 gives me between 45% and 55%. The rule is worst at sea level, and gets better as the altitude increases.
The rule of 48 is not very good. It is about as accurate as saying that on average, the sun's position is in the south (for us northern hemisphere folks). So if you head for the sun you'll be going south. We wouldn't propose that as a means of navigation, and we shouldn't propose the rule of 48 as a substitute for a power setting chart.
If you have an O-360-A series engine, and an MP gauge, I created an Excel spreadsheet that replicates the Lycoming power chart:
O-360-A power spreadsheet
If you have an angle-valve IO-360 engine, you want:
IO-360 power spreadsheet
You could produce your own power charts using those spreadsheets. With a FP prop, it is relatively easy to set a desired MP. The rpm will vary with your speed. You could create a chart that had sections for various altitudes of interest. Each section would have a table with MP and rpm values, and the resulting power.
Or, you could use the O-360 power chart that Larry Pardue created, using my spreadsheet:
Larry Pardue's power chart