Hello,
I make quite a few quilts that are on point.
You will need formula #1 for the side setting triangles:
multiply the finished block size by 1.414 and add 1.25" for seam allowance. Round up to the nearest 1/4 inch. In your case, you will cut your squares at 18.25, then cut each diagonally twice to get 4 side triangles.
You will need formula #2 for the four corners:
Divide the finished block size by 1.414 and add .875 for seam allowance. Again, round up to the nearest 1/4 inch, In your case, you will cut TWO 9.5" squares, and cut each on the diagonal ONCE for a total of 4 corner triangles.
these measurements are generous and will allow you to trim everything down after they are sewn.
happy sewing,
Jennnie