Only if you are able to hit with the sweet spot of your putter every time, you can be sure that a consistent stroke generates the same roll every time. You may be shocked when you try this exercise, that you are actually making contact all over the place with your putter. But do not sweat it. Once you have tried this exercise you will improve your accuracy fast because it makes things visible you simply have not noticed before.
You do a normal putting routine of your choice, but you stick two medical strips on the clubface of your putter. Place them around the sweet spot of your putter approximately half an inch apart.
The sweet spot is usually exactly where the marking is, on top of your putter. To find it hold your putter loosely in front of you, in the same angle as when you putt. Then take a tee and knock with it on the clubface. When you hit the sweet spot, the club will not wobble and produce only minimal vibration. It is a good thing to try at least once.
You get, see and feel first hand how much impact the sweet spot actually has on your putting swing. With the medical strips on the clubhead start putting away. You get immediate feedback, if you do not hit with the sweet spot. There are also small metal pieces you can stick to your clubhead. These do basically the same thing by deflecting the ball, if you hit off-center. If you miss hit a lot at first, swing slow but with a smooth acceleration through the ball.
After a while you should get a better feel for your putting stroke. If you want to, you could even take your modified putter with you on a round of golf. The added pressure will make you focus on your putts even more.
Place two medical strips left and right of the sweet spot on the clubhead of your putter (approximately half an inch apart).