I re-upholstered some furniture for my wife, (before we were married) I used a drafting trammel to get the curves just right and drew the stitch lines on the back also. I had all my pieces listed on my note paper. I had bought one of those cheap sewing kits at walmart that had a needle threader, cloth tape thread. etc. and had it in a drawer in the sewing table. I picked up the cloth tape to check the pieces to lay them out to start sewing and they did not match the notes I had made when doing the layout. THE INCH LAYOUT ON THE TAPE WAS OFF ABOUT 20 %. I took a second look and said those curves are more than (x) inches apart. Metric side was ok.