I tiller with a pull rope but I also have holes drilled in the wall board of the tiller tree at every inch down the tree and I use a smooth shank bolt to hold the bow string down while using the gizmo.
How long is the string you have on the bow in that picture?
I start out tillering with a string that just barely slides into the string grooves and lays snug against the belly of the bow.
Longer strings lie and don't give you a true indication of the tiller.
Once the tips are bending down about 6 inches, I brace the bow to 4 inches and keep tillering.
When the tips are bending down 9 inches, I brace the bow to 6.5 inches and finish the tillering process.
So far your bow is looking nice.