
I've been using a home made version of that for a while now. In the pick above I had some hop bags with pellet hops in there and this was how much I had left over after I took the hop bags out. Probably somewhere between 1/4 and 1/2 gallons. I give it a good stir after flame out to get a nice trub cone, and the more trub you have the less liquid you leave in the keggle.
If I use pellet hops I put them in a hop bag and if I use whole leaf I just drop them in and try to scoop as much as I can out with a strainer before draining.