Seeing as this is about arches, this is where you need to start. If you don't understand the layout use and fabrication of arches after reading these, go back to straight lines.;http://www.thisiscarpentry.com/author/wm-todd-murdoc/
As Brice pointed out, the length of the radius in a simple arc is all that matters. As long as everything is set up on the centerline, life is good. The biggest issue is the pivot point and router diameter. This affects the arc, the size of an arc is different on the inside and outside of a cut by a 1/4'" diameter router bit.
If you want a bracket to set the pivot point and the router bit inline, use the gauging bracket from the LR 32 system. It will clamp to the rail and the spring loaded edge stop pin works as the pivot. That is fine for small arches, very hard to keep it from pulling out of the hole on larger arches.
They pivot through the rail is a 5/16" x 24 bolt 1 1/4" long, so the rail rotates on the shoulder, not the threads. I use a nylock nut, set to "snug" the rail.
The stops for the router are the rail stops that come with the saw. I set up to demonstrate how to make this process work, then realized I only own 1 stop. DOOOOHHHHhhhhh (Now if that Tom guy from Tool Home could remember I forgot [and never told him I needed] to order another stop, that would be customer service.) Once set with 2 stops the router will not move, but it will lift straight up.
I have routed 1" cherry with this set up. 2 passes. No issues what so ever. Seeing as it is easily adjusted, you could make a clearing cut, then adjust to the proper dimension and make the final cut. Biggest issue I've had with real long radiuses is the surface your working on shifting a little. That is why this is on the floor.
I neglected to mention that the vertical flex in the guide rail will affect the cut. You ned to support the intermediate sections of the rail with material the same thickness as the pivot support and the piece being machined (these should be the same thickness). I space mine about every 2'.
Sorry about the lousy pictures, all taken with my iPhone. Sometimes I can't hold it steady enough when I click the shutter.