Does anyone know how to compute fuel consumption when the engine is on idle given the Engine displacement and RPM.

Tried searching the net and the most i can get is:
Typical Engine Idle Fuel Consumption:

500-700RPM >>>> Half a Gallon/Hour (1.8918 liters/Hour)
900-1200RPM >>>> 0.75 GPH (2.8377 LPH)

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FC on a 2-second keystart is about 0.0024/Gallon (0.0091/liter)