This is a graph shows flow rates for single outlet 3" and 4" Flouts. The column shows maximum  flows for a 3" Flout. The flows are the maximum at turn-on measured at the vent. For reference, the upper curve of each pair shows flow thru a simple orifice. Lower curve shows flow through the Flout.

 The average flow from any Flout is the same as the maximum flow of  one with 1/2 of the original drawdown. Example: What is the average flow of a 3" Flout at 36" drawdown? It is the same as the maximum for a 3" Flout with 1/2 the drawdown (18") The average is 115 gpm. 

Multiply flow by number of outlets for multiple outlet Flouts.