Evening Wes,
Its essentially an educated guess for an engine you havent Dynod as its based on the fuel requirements for the horsepower developed by the engine.

The brake specific rule of thumb is that your average piston engine will consume 1/2 a pound of fuel per bhp per hour. This is termed .5 BSFC. The best brake number always occurs at peak torque where the engine is opperating most efficiently.