Here are a couple approximate ways to calculate fuel flow.
http://en.wikipedia.org/wiki/Fuel_in...rom_pulsewidth
I should consider (Fuel flow rate) ≈ (throttle position) × (rpm) × (cylinders) for my bike.
Nobody I know has (or should) reverse engineer the SG, so it is anyones guess exactly how they do it. But there are plenty of threads here and on the net that discuss it and go into varying degrees of accuracy and complexity.