For our races, we'd fill the Work & Play all the way with over 100 gallons. Although we didn't use it all, we used well over half. Then I'd open the drain before we headed home so its was really getting low by the time we hit the road.
On a side note. I have a 225 gallon tank sprayer I use for my landscaping business. It sits in the bed of my F350. Its far easier to haul around completely full versus half full. When its only half full, its amazing how much it will push and pull the truck from all the sloshing back and forth.