You can use two methods in determining the proper length for your putter shaft:
1) A2 x B2 = C2. A is the distance from your hands to the ground while in a neutral set-up position. B is the distance from where the bottom of A strikes the ground to the target line or your eye position. C is the length of your shaft.
2) While in a neutral set-up position, rest the end of your putter shaft against your forward leg. Place your forward arm better known as your directional arm behind your back and make 3 putt strokes with your rear arm or better known as your power arm. On the 3rd putt stroke, use your directional hand to place the grip in the palm of your power hand. Mark the spot where the thumb of your power hand rest on the grip. Measure from the bottom of the putter to this spot and add 9 inches. If it agrees with the formula A2 x B2 = C2 then that is the length your putter shaft should be.