I'm not sure about that being the right math for either the "real world" or quilting. Typically to sew together a medallion, you would sew the pieces for one half together, then sew the pieces for the other half together & then seam the two halves together. Maybe try doing one with that extra piece #41 cut & see if it's necessary. If not, I'd go with 40. I'm with PaperPrincess. Larger seams should result in a smaller diameter circle. 41 pieces cut on a 9 degree angle shouldn't make a true circle unless you fail to maintain each of the angles in exactly by a fraction of a degree when seaming them together.