I hook on where the fulcrum of my string hand(middle finger) will be. To find this, I make some measurements on the wall behind my tillering tree. I measure 3/8 from where the shelf will be and make a mark. This mark mimcs where the bottom of the nock point will be.
Next, since my arrow nocks are 1/4" wide, I measure back toward the handle 1/4" and make another mark. From here I measure half the width of my middle finger and make one final mark. This is where I hook onto the string because this is where the fulcrum point of my string hand will be.
I also draw a plumb line down from this point toward the floor. This is the line the fulcrum point on the string will follow if the limbs are synchronized. If it doesn't follow the line, it is being pulled toward the stronger limb and I weaken it.