There are two things that determine your ball position.
1. Target Line
It is generally recommended that the position of the ball should be under your eyes or for your eyes to be over the target line.
Standing too close to the target line will cause your eyes to be outside the target line and your path direction to be out-to-in, sending your ball to the left.
Standing too far from the ball will cause your eyes to be inside the target line and your path direction to be in-to-out, sending your ball to the right.
For example, most golfers, because their putter is too long, have their eyes too far inside the target line. This makes aiming more difficult as their visual perspective is skewed. In other words they are seeing the target line at a diagonal rather than parallel. This causes the aimline to readjust to the right side of the target.
2. Eye Position
Your ball position is relative to your eye position.The optimum ball position is typically around 2 inches in front of your dominant eye position.
A common test is to take up your neutral setup position and then drop a ball from your dominant eye. The ball should land just behind your ball on the ground.
Some instructors recommend that the putterface impact the ball at the bottom of the arc. Their argument is if the putterface impacts the ball on the upstroke it will go a little to the left because the putterface is closing. However, releasing the putterhead eliminates this concern as the putterface stays square to the target line.
There are others who advocate positioning the ball forward in your stance. They believe putting is similar to a full swing in that you want to contact the ball slightly on the upstroke. If your ball position is too far forward, the bottom edge of the putterface will catch the top quadrant of the ball resulting in an inconsistent roll. It is also likely that you will make a longer backstroke, but stop your forward stroke too quickly.
The only place everyone agrees that it is not advisable to place the ball is in the back of your stance as this will result in your putter impacting the ball in a descending arc before your stroke has bottomed out.
As I stated earlier, the recommended distance for positioning your ball forward of your dominant eye position is one to two inches. This allows your dominant eye to be positioned behind the back of the ball.
HINT: Placing your putter up against the ball runs the risk that you will catch the grass with your putter as you start your backstroke. This is because your putter is now forward of where your stroke bottoms out.
Instead, hover your putter so that is not pressed down in the turf or lift your putter at the start of your backstroke.Remember to leave a small gap between your putterface and the ball.