Pretty insightful question for a cat guy! I’ve got to say I agree with the “let the fish tell ya” answer. I’ve had times trolling 4′ of water where they would not touch it if it hit bottom, other times where it had to be slamming. THe action of the lure also has a lot to do with it. Tight wobbling lures like shad raps I tend to start deep and move up, wally divers and others with a more aggresive wobble, I try to keep off bottom to start, but will regularly change patterns until one becomes apparently more productive!