Good advice above.
I don't like that tiller sticks or trees, I don't use it anymore except for measuring drawweight. I do all my fine tillering in front of a huge mirror in about 5ft distance. My experience is having the bow in hand and pulling at the right place shows a really different shape than on the tree. And you can watch the movement of the limbs. Another thing is the feel you get, I mean the movement of the whole system. Does it want to twist, does it want to upward or downward.