Place the 'Start Line Trainer' on to the ground aligned in the direction that you want to roll the ball.
Take x2 TRUVIS balls positioned so the diameter of each ball is directly over the inner holes which are designed to form the gate for the 3 different width settings seen on the graphic below 1, 2 or 3.
The gate setting closest to the ball requires less than 0.5 degree of accuracy from the putter face angle at impact. The second closest requires less than 0.75 degree of accuracy, whilst the setting furthest away requires less than 1 degree of putter face angle error.
On a straight 10ft putt you will need to develop accuracy of less than 1 degree of error in your balls starting line to be successful.