How much water to ad to black tank when starting out.?

OP, presume you're in the park with the neighbor? Whatever you're doing must be working, so what doo you doo?
People fail to realize that campers on the move don't hardly ever have issues because they are on the move.
But if you're parked, it's like a portapotty. Even though there's liquid, mount poosoovius is still a real concern.

Use some common sense and don't just ASSume that once hanky the christmas poo disappears down the drain, he's magically gone!