Arc of a specific radius issue

Im trying to create a arc between two fixed points. The arc needs to have a specific radius. This is a guitar fingerboard. I worked on the bottom end of the board first, creating the two lines a specific distance from each other. I locked the end points and created an ac, then set the radius. It worked perfectly (the arc was the bottom of a circle, think smile.)

I then went to do the same thing on the top end of the fingerboard. Same process. Two lines, locked end points, created the arc. But when I go to set the radius, it creates a circle with the piece I want missing.

Btw- I’m on my iPad.

Here is the bottom arc finished

Here is the top arc with issue

Here is a video trying to add the arc. It appears that Shapr3d is making a giant arc with the right radius, but making the points the bottom of the arc/radius rather than the top. Not sure how to fix it.