I am having a pool built right now. It has an attached spa. When I fill the pool / spa, I can determine the exact gallons of the spa by measuring how long it takes to fill and knowing the gpm from my hose. Later, can I then drain the spa, fill it with the pump making sure it is sucking from the pool drain and returning only to the spa and be able to figure out the pump gpm? I know the head might be slightly off, but do you think it will be close enough?