So when sizing the supply lines for some plumbing, you have to calculate fixture units and then make sure the supply can support the max demand, but is that realistic? It seems pretty case by case, like a house with 4 bathrooms would be very unlikely to ever have all fixtures in use at the same time. The main problem I see with this rule is that it could be very energy inefficient, because it requires larger supply pipes, which means more time and energy to move hot water to fixtures? And what's the tradeoff? So that for the unlikely event you are using every fixture at the same time, you get good flow?