Don't have it with me now, but there's a table in the little black book named Pocket Reference that allows you to estimate GPM from an open pipe by measuring the horizontal distance it takes for the stream to drop x inches.
EXCEPTIONAL little book.
It does not work like that! Use the B&G System Syzer. Example: 45 GPM in a 2 inch iron pipe, within normal design parameters can pass 48 GPM with a loss of 4.5ft (aprox 10 PSI) per 100 ft of pipe. I looked up a pressure loss table. At 50 PSI ( pressure loss) per 100 FT, it is a flow of 116 GPM which is entirely unrealistic. It surpases the maximum recommended velocity of 10 FPS! The lenght of the pipe is the most important factor, if you are working either with a pump or city pressure on an open system.