Does the tank or pump apply any pressure in this system? If it does that pressure can be added to the npsh which will make you pumpless likly to cavitate. If the tank is at atmospheric pressure just enter zero.
the net change in height is the net change in height between the tanks/pumps lowest product level and the height of the pump we are currently calculating the NPSH for. so if the pump is below the product level the Net change in height would be negitive but if it was above the product line it would be positive.