I was thinking about a pump curve looking thing for flowing wells. I would assume that somewhere someone has created such a chart. In fact, I bet there has been many a study on it. I was just hoping for a rule of thumb.
In Montana, there is a spring called big spring. This spring flows 40,000 gpm if I remember right. It is a result of the Madison formation breaching the surface. One of the local economic development people (USDA) got involved with a hydrologist who thought that it would be possible to intercept this water with a well drilled into the Madison farther up the hill. The goal was to drill a 10 water well 2600 deep. We placed a bid on it. 3500 feet and $600,000 later, we ended up finding that the Madison was not very thick where they wanted us to drill. As far as I know the well now makes about 10 gpm. We also hit oil on the way there.