My goal was an arc that was 35" wide and had a rise at the midpoint of 4.5" . SketchUp made this child's play to figure out. Simply drawing the curve a nd right-clicking on it got me the radius (36-9/32") with no calculations.
But then I had to implement it, of course. That went well also. I made mark s on my 1x12 oak 35" apart and a mark 4.5" inches in from the edge at the c enter of what would be the arc. I then turned the trammel upside down and m easured along the center line 36-9/32" from the edge of the router bit and marked the spot for the pivot pin.
I intended to use a 10 penny nail for the pin, but it fit just slightly loo sely in the hole I drilled. This probably would have made almost no differe nce, but I found another item that fit the hole perfectly - the drill bit i tself. The smooth shank, of course.
I made 5 or six passes, using the stepped dial on my plunge router to set t he depth for each pass. In the end, the curve passed exactly through the ma rks I had made originally. Success!
Thanks to all who offered advice.