skdesigner,
I do not know it is that simple.
If this is light hazard, then the sprinkler flow would be 0.1 gpm/sf, measured over 1500 sf. This would give you 150 gpm for the sprinkler load. Adding 50 gpm would give you 200 gpm. Minimum pressure for sprinklers is typically 7 psig. So the water supply would have to be able to provide 200 gpm and overcome whatever pressure drop which would result in 7 psig at the most remote sprinklers.
If the hose valve is at around 4' off the floor and the sprinklers are at 10' off the floor, you gain ~2.6 psig back - so the minimum pressure at the hose valve would be ~9.6 psig. Depending on where the hose is at in the system, you will gain back whatever pipe friction lost flowing to the sprinkler. But even if it was 10 psig, would ~20 psig be enough to flow through a hose connected to the hose connection?
If this is a commercial building with a fire department pumper connection, I do not know if you need to know the pressure at the fire hose connection. The fire department will provide the pressure with their equipment.
If there is not a fire department connection, I would likely fall back on 4.5 bars which NFPA 14 requires for a Class II standpipe (1.5" hose).