I'm so glad I found this link. I had the same problem with a Dresden Plate I made two days ago...really thought i had been careful with my 1/4" seams. Anyway, the ring was a tad bigger around the middle than the outer edges, so it was kind of wrinkley. Although all the hints are useful, the idea of assembling quarters and then squaring them up sounds like it will make it foolproof.
Thank you all!