How much liquid will cover the bottom of my black tank?

After dumping, I have always gone ahead and put in whatever black tank treatment we're using, normally Odorlos, and add about 5 gallons of water. And let it slosh. If it's a longer period between trips, when we start out again, I'll go ahead and add some more treatment. We're minimal with the FW tank - since we rarely use it - but generous with the black tank water.