Hot to convert drainage fixture units to gpm?

7.48 Gal (1 CU FT) = 1 FU Or 1 GPM of pumped drainage = 1 FU

The above equation that was submitted makes no sense whatsoever. Conversion of Fixture Units to gpm is non-linear; gpm per FU decreases as the total number of FU increases. I am a civil engineer, not mechanical engineer, but I know there are resources available either through International or local building codes that guide the designer.