For the dress above, I used my new quilting ruler (Easy Circle Cut™ by Sharon Hultgren). I needed a 7-in. diameter finished circle for a 24-in. circumference shirt. It turned out to be perfect!
If you do the math, you might think it wouldn't be large enough, but you are going to sew 3/8" or 1/2" from the edge of the 7-in. diameter circle, so it is going to get larger--don't make the mistake of cutting that circle too large!
I didn't get "exotic" and do the math; I simply inserted my plastic measuring tape in the groove of the ruler and came up with 12-inches on the 7-in. template. The top was 24-in. circumference, so it worked perfectly. (If I had 'done the math', I might have screwed up and used the 8-in. template, but you are increasing the diameter to about 8-in. when you take away the seam allowance.)
(Get it when they are 50% off!)