
Rather than repeat this problem I strung 11s then did two rows of 11s, moved my thread into the inside and did 2 rows of 15s. I then added a third row of 15s and when completed I did NOT step up into the last added row. I remained where I was and did 1 row of 15s by "stitching-in-the-ditch". From the stitch in the ditch 15s I did one more row of 15s and 2 rows of 11s - see right side ring above.
I now have a perfect circle and by stitching-in-the-ditch the centre is perfectly round, rigid and firm.
I then did some extra peyote rows of size 8s, on each side of the ring. I then zipped the two sides together.
On the left you have a side which is a little concave but certainly not a problem.

On the right you have the opposite side which is a little convex, also not a problem - in fact making it great for further embellishing.
I will be making my seed bead rings this way in future. I hope you can benefit from this little experiment.