I used to have a slide chart that gave gpm through a level section of pipe (1/2" - 6") by measuring how long the exit stream was when it had dropped 4". Imagine a framing square with the blade on the pipe and the tongue hanging down in the stream. Slide the square out until the stream is hitting the 4" mark on the tongue - read length from end of pipe on the blade of the square. The slide chart would give the gpm based on various pipe sizes. Has anyone seen one of these charts? Mine was from 30 years ago. Maybe by a water pump company? Can anyone give the equation behind this method?