This is my official first bolster pillow. I had attempted one the week before this one, and it did not work out well. I used the Pi method for determining the circumference, and it did not work. Let me just give you a word of warning. Pillow inserts are not perfect! My insert had a diameter on one side 1/2" off from the other, so when I did the calculation, I used the smaller side. I hadn't checked the other side, so I didn't know there was a difference. This pillow insert was off one full inch from one side to the other. I chose to measure the circumference of the insert itself to be sure I had the right amount of fabric. Your calculations are only as good as the pillow measurements.
I chose to use a lapped zipper on the pillow versus an invisible or exposed. There wasn't a lot of extra bulk in the seam, so it worked well. If you have really thick fabric, this may not work especially if it is piped as this one is.
Because my pillow insert is off in its size, I think it's distorting the shape of my cover. :( You may want to make adjustments down in the size of your pillow case to fill in any gaps. This particular customer wanted it true to size, so it isn't filling out all the spaces.
It was also very hard to get perfectly round circles despite my best efforts. I think in the future, I will interface the circle in hopes that it will retain its shape when stitching it on. I'll let you know how that works. I'm still working on another bolster. I cut is too small using the Pi method, so now I'm trying to decide how to fix it.
Let me know if you have questions. And as always, thanks for stopping by!