Trying to find the length of a curve.

Hello everyone. I'm trying to find the length of a curve from the center line to the distance. I need to fit a bolt bar at 21" from the center line. I need to find the length of the curve in inches or degrees or both if possible. Theres a link below to a picture of what Im trying to do. Please help and thank you for your time.

This is the formula I have: Distance divided by radius multiplied by sin multiplied by one degree.

http://i815.photobucket.com/albums/z...ezerg/Sin1.jpg