When I did it on this quilt:
http://www.quiltingboard.com/picture...r-t189285.html
I drafted the circle out using a metal tape measure and my DH help. I had him hold the zero end at the corner of a big piece of cardboard. I held my sharpie pen at the measurement I wanted then drug the pen and tape measure across the cardboard with DH holding the zero at the corner. I had a wonderful quarter round. Then I did it again for my outside arc. I cut out my template and used that to cut out my fabric. don't forget to include your 1/4" seam allowance! I sewed the four units together then carefully pinned to my center star and prayed it would work! It did. Not sure I would want to do it this way again but it turned out great.