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I found an online converter that gives me a diameter of 9.2" including seam. Will see how that goes tomorrow. Starting from the base makes more sense, I'll try that next time.
moocher-
But you've already closed the seam on the tube no? If so, you just need the finished circumference of the tube. If that is 28" it should give a disc diameter of 8.91". If you then sew a half inch seam all the way around, the finished disc when you turn it right way around should be pretty much an inch less than that in diameter.
Black_Rainbow_Project
The circumference of the bottom disc also has to be 28"
Once you have set the size of the tube it can't grow.
If you make the disc bigger you will just end up with excess fabric around the edge when you sew to the quarters.
Ideally you need to set the size of the disc as you want it to end up after completion. Then add your seam allowance to the outside of it, then measure the circumference. The tube is then that size, plus whatever seam allowance you want to close the tube on each end. Because you have a closed tube already, the disc can't have a bigger circumference than the tube.