I figure with 25g of ro/di, topped off now and again with 10 ro/di gallons, anything that sinks to the bottom will just get stirred up and mixed eventually. So I just put in about a pound, and will wash out the tub periodically. Hope there will be no problem with this imprecision. It is hard to be precise with a garbage can.
The grid floor holds my topoff pump up and well away from the slurry in the bottom, and a Brute has a 2x2" depressed ring around the outside base, so that is where the slurry tends to be. So only the filmy stuff can get up through the grid [about 15" up] to reach the topoff pump.