The notch is at 20mm. Centered to the wing points. You can center it where ever you like. What really matters is how evenly it connects to the wings.
In my actual model I was grabbing the wings a bit below center. But I also dug below the line to get a full wing opening. It was hand-made, so it required an iterative process until I got it right.
It can only be perfected with a CAD drawing with gears, which Xigfrid doesn't do.
Overall, this new gear fits the 20mm distance centered at the notch. It is also balanced on both sides, flat, and made with one piece with clean notches. All of which will make it many times better than my hand made one.
Mine also got bent during testing multiple times, so while it started out smooth, it ended up jerky. There are 18th century watch makers turning over in their graves.