As you are well aware, the depth of the ball is more important than speed or distance the lure is from the ball. I rarely go back more than 15 feet and stop the ball 2'-5' above the walleye. Keep changing the depth until you find what works. On slow days when the fish just don't seem to want anything you throw at them run the ball all the way to the bottom, then bring it up a foot or so and watch your fish finder. When you see fish suspended, jerk the line loose and jig your lure through the fish. I've gotten many a walleye on slow days with this tactic, especially with spoons and shallow diving cranks like a Yozuri minnow.