You know the static head at 50 psi on one hydrant, therefore, that pressure will be on straight line on a semi log hydraulic graph where Q^1.85 power is represented on the absissa. Q is in GPM and the ordinate is the pressure in psi. Assume at least one flow rate in GPM between that hydrant and a next one, then, knowing pipe length including equiv. length as well as pipe diameter calculate pressure drop via Hazen-William formula with a C value corresponding to piping age. Plot the friction loss as a straight line on same graph starting the line at 0,0. Subtract static head line from friction line and the resulting line which probably be no longer a straight line will be you water supply at the 50 psi location. Since the city will probably require a minimum 20 psi at hydrants at that 50 psi location you can determine that approximate amount of water at the 20 psi residual pressure. Note if there is significant difference in elevation between your two hydrants, the 50psi line will have to be adjusted accordingly.
