the only proper way to determine over flow amount is to measure it. Get a 1 gallon pitcher and have the return fill it. count the seconds it takes to fill and then convert that to gph... Manufacturer numbers can't take all the head loss and like in to account. Also pumps slow down with out cleaning. The flow through rate in the sump is never specified by skimmer manufacturers that I'm aware of, do you know of a skimmer that specifies this number?