http://www.homebuildingandrepairs.com/framing/index.html Click on this link if you're looking for more information about building arches, roof framing and construction repairs. Here's another simple and easy step-by-step tutorial that will help do-it-yourselfers and carpenters figure out the radius for a segmented arch used on the interior and exterior of homes and other buildings. Complicated construction math formulas simplified.
https://youtu.be/lt21XbGAJN8 Check out this old time carpenter trick for arches.