RJB32482,
The governing equation for most common fluid flow problems is the Bernoulli equation, which is a mechanical energy balance involving kinetic energy (velocity), potential energy (pressure and elevation), and frictional losses. If your header pressure was at 0 psig, you would still have flow to the atmosphere because the header has 15 feet more potential energy than the discharge. I converted this potential energy from an elevation basis to a pressure basis by the conversion factor of 2.31 feet of water = 1 psi of pressure. Since the potential energy due to elevation is causing or adding to the flow of fluid I added to the potential energy due to pressure. It also is called "head". You have 80 psi of pressure AND 15 feet of head.
If you had 15 feet of vertical rise, I would have subtracted, but that was not the situation.
Basically, I did the Bernoulli equation in my head and entered the resulting overall potential energy in terms of differential pressure into my handy dandy spreadsheet and trial and error solved for the flow rate which generates losses that equal the dP. I think it took at most 2-3 minutes time.
Good luck,
Latexman