whitepine and m-faye, if you started with a 12 1/2" center and put a 4 1/2" round on it, it should now measure 20 1/2". Next round you'll have to use a cornerstone for a fill-in and then it will be back on track again. When you start with a 12 1/2" or 18 1/2" center you just have to deal with that uneven number somewhere along the way until you can work it in. I'll see what I can some up with for you as an option you might like, for right now your centers should measure 20 1/2" for what you started with. If you want a plain border to block your center, put it on now. Strips should be cut to 2 1/2" wide and sewn on, then your centers should measure 24 1/2" (if it measures 21" now, that's okay) as long as you trim it to measure 24 1/2"...