Dome - Can I determine the Radius of a dome?

[Davepam]

I am trying to setup a dome of about 1/4 inch wide, 3/8 deep and about 2-1/2" tall (On the computer screen). The arch is to curve in the 2-1/2". I would like the ends to blend into the 3/8 depth bottom plane, And the top of the arc to be even with the original surface of the wood. I am gussing at the radius - Maybe 1-3/8" to 1-5/8" ( I just need the concept). I get the cut depth, but the "100 to 200" is driving me nuts. There needs to be a way to set in a radius. Thanks for any experience on this.
DaveKat

[Greybeard]

Dave - could you post a drawing. A simple sketch would do.
John