After I had placed all the petals, I found that they extended in places, more than a little past the 10 inches requirement at the 'cut' edges. One petal I left and the other I adjusted about 1/4 inch.
A piece of canvas was used as a backing and then a binding of single thickness was attached as close a possible to the flower center edges. I couldn't have made tighter if I tried. Another line of stitching on the front surface of the binding should finish this first one, but I decided to glue it in place on the backside before top stitching it. One less thing to have to manipulate in these tight quarters.