Lycoming Horsepower Calculation?

I'm looking at the Lycoming Operators Manual Performance Data Chart for for
the O-360-A1A....(page 3-51 in my manual). And after looking at this for a
few min i need a beer.

Does anyone know of a formula that covers this chart calculation...?

I want to find the horsepower based on altitude, rpm, manifold pressure,
and air inlet temp...just like the chart shows.

I have a homegrown engine monitor that i fly with and want to add the calc
for percent horsepower.

thanks,

Mark
N515MD Mustang II
Raleigh, NC.