1. P1 is the minimum pressure that you want to maintain at that location. It is System Pressure if the tank is located at the booster pump, or the pressure that you are maintaining at the top of the building if the tank is located there.
2. ∆P is the expected pressure change. For a skid-mounted tank with constant speed pumps, it is the pump’s rise to shutoff, plus any typical additional suction pressure. For a constant speed system and a remote tank, it is the expected friction loss in the piping. For a VFD pump system using the 10 psi shutdown boost feature, add 10 psi.
3. The number in white is the amount of storage expected in a 100 gallon tank. Or the percentage stored in another size tank.