This will be the tale of 3 borders. So sit back a minute...
1. When I got my center, I was pleased as punch & knew exactly what I wanted to do. Then came the math... That idea never really gained traction for all of the math. But in my mind it was pretty cool.
2. Border 3 also included math (Which one wouldn't, right?) but this time it would add too many inches to the project - but based on the finished about 20 inches parameter & for the center itself.
3. Border 3 - you can see above how I cut with templates & how I auditioned it. :o)
I pieced the corners to give me the size needed for the sides. I then trimmed the sides down - for me this is usually easier that sewing skinny strips. & My initial length measurements were only 3/4 of an inch off - easy to unpick & trim to the appropriate size. YAY! Everything worked so well & I am quite pleased with how it looks - especially next to the center.