Ross Jan 10, 2017
Not sure. Everyone is a little different and depends on the width of the stance when/where impact will be. You want it to feel solid and just past the bottom of the arc on the up swing for end-over-end roll. The worst is to hit down on the putt (ball hops). At the bottom, the ball slides before rolling. Again, I think slightly on the up swing for perfect roll*.
* This also assumes your shoulders are parallel to your intended path, and the putter is 90° to that path. Dave Pelz had an easy way to see if your shoulders are aligned, but letting your arms hang and pointing your thumbs towards each other, then rocking your shoulders. This will show your your shoulder path… and it better be parallel to the target line.