I used to use a mathmatical furmula for figuring the radius of an arc. I came across it years back in a Fine WoodWorking issue from the early 1990’s.  I believe it was located in the methods of work section. I’ve long since lost that issue and I haven’t used the furmala for about five years. The furmula used the height of the arc and the chord length. then it was a basic algebraic process. but I just can’t remember it. If any body knows it or could look it up in their back issues (the cover of the issue has a woman clamping up a small table) I would really appreciate it.