Here’s a tip for marking large arcs.
Instead of using a beam compass, or simply a nail, string, and pencil, use a thin piece of stock or rip a thin strip. I like to keep thin strips ripped from 8′ stock because I design many projects that call for 2 1/4″ parts. Once a 1×3 (which measures 2 1/2″ wide) is ripped to 2 1/4″ I’m left with a 1/8″ strip. Keeping 8′ lengths of these strips gives me plenty to work with in nearly any size arc I might need. And of course, I cut some of them shorter – 8′ gets a bit unwieldy.
I mark my end points and the peak of the arc on the workpiece. Then I drive brads near these points (allowing for the strip and pencil thicknesses). I fit the strip against the end points, then “bow” the strip to reach the arc peak. Then I simply use the strip as my guide for marking the arc.
I make sure to apply even pressure on the strip at the arc peak, and I don’t put any additional pressure on the strip when I make the mark.
It will take some practice, but it creates a quick arc when you’ve got the hang of it. Then all you have to do is have a steady hand when cutting the arc.