Thanks for everyone's reply's. I ended up using a tape measure as a trammel. I drilled a hole just big enough for a nail at 1' for the pivot point and another at 21', to make the 20 foot radius. I used 1 sawhorse 90 degrees to the timber with a board clamped to it at the height of the timber. I then set up the timber on sawhorses 20' away and moved it back and forth to find my 2 points where the arc would enter and leave the timber. Once I bulls-eyed these two points I put a pencil in the hole at 21' and drew the arc. The whole process only took me five minutes and I found it to be quite accurate.
