So using a grease pencil tied in a loop at the end of some Mason's line, I anchored the other end around the rear axle adjusted the length of the line until it started where I wanted it to. I then drew an arc to the bottom rear of the fender. Some quality time with a jigsaw made the arc happen and a little help from my heat gun resealed and smoothed the edges... Happy with the result, the arc of the rear fender follows that of the rear wheel and tire!