Originally Posted by Jan in VA

It seems like you have answered your own question with the measurements you gave.
1. The corners are 6" square approx? Cut a square for that corner at 6" + .5" seam allowance + a couple of inches for 'extra' = 6.5" + 2" = 8.5" sq. for the corners.
2. The triangles between the points are slightly more thoughtful because you need to put a straight grain edge, not a bias edge, on the outside (longer edge) of the triangle for stability. So, using your stated measurement of 8 3/8" cut 2 squares that are approx 10" square. [I got this measurement by adding an extra 1 5/8" to your own measurement, just rounding it up to something more manageable that that "eighths-of-an-inch" stuff and giving you some extra to be sure. ] Now when you cut the squares across ONE diagonal (which will give you a total of 4 triangles), you'll have those outside edges on the straight grain.

** The reason I add "extra" to these setting triangles/squares is because I want to be able to trim the finished block to a proper size in a way that will NOT lose any points. I allow about 1/2" rather than 1/4" seam allowance so that the block appears to "float" away from the seam --- this makes for a more crisp look when you are done; the points do not look crowded by the adjoining sections of the quilt. The "extra" also allows you to make the block perfectly square when trimming.

Be sure to share the finished quilt with us!

Jan in VA