I don't use a line.
I would suggest a drill which I have used recently. You can do this at home, best in the yard, but you can do it if you have a large open room or basement. Cut a piece of cardboard or thin wood the size of the hole. Place it on the ground/floor, put your golf ball out 5'-6'. (beyond your vision when looking directly at the ball) using a ball -- line up to putt. Then close your eyes, turn you putter upside down and see if you can put the butt of the shaft on the hole. Move around the hole same distance and repeat. Expand the distance. What you are doing is learning how to accurately visualize. Move this to a practice green when it is relatively empty and you will deal with slope and actually putting. When you are practicing on a putting green do this after 3 or 4 putts.
I learned this from another golfer last year. He is one of the best putters I have every played with. He became a regular player at my home course last year. I noticed how he warmed up on the practice green and asked him what he was doing. He explained, he read an article about this in a golf publication years ago and always did this in his warm up & practice. His point was putting is two distinct actions, right line and right speed. Mentally you won't tend to do both well unless you concentrate on one at a time. Establish your line, and then totally focus on the stroke correct for the distance and break. You must visualize your distance correctly being confident of your hole location as you stroke the ball. His point was some people actually putt better looking at the hole, but the reason is they have not worked to develop their ability to visualize. I was not a bad putter, but it made sense and I have found it has helped me improve much more than I thought. Really did not help in the short putts, but I have seen solid improvement on the longer and especially breaking putts where speed is so important.