To answer the question at hand requires an extreme high level of math and physics. Here goes:
One crams the hose, previously connected to the water source spicket, into the filler port. Please remove the cap first. Makes things much easier. Turn on the water, about halfway, sit back, sip a cool fermented beverage of your choice and wait for the water to overflow. It will! Using this method, there is no way to overfill the tank. Then go over to the valve and shut off the water, remove the hose from the fill port and replace the cap. In that order. Otherwise, one gets their shoes wet. You now have a freshwater tank that is filled. Disconnect the hose from the site faucet, drain the hose and return it to your storage bay. It is now time for a refill on the fermented beverage.
Bob