People ask me all the time about ball position with their putting. Should it be forward in the stance or back in the stance? The most simple way to find the proper ball position it to look at your stroke. The putter goes up, down and back up again. The most neutral position is when the putter is at the bottom of that arc.
That will likely match up with the zipper of your pants or the buttons on your shirt, and that can move based on your weight distribution. One thing to keep in mind is that you don’t want the ball in that position, you want the putter to be there so that you’re striking the ball at the bottom of your arc.