HALF SQUARE TRIANGLES FOR QUILTS SET ON POINT

For side triangles:

Take your finished size x 1.414  and add 1.25 for seam allowance. 

If your finished size is 15”, then

(15 x 1.414) + 1.25 = 22.46    Cut a square 22.5 inches.

Slice on both diagonals.

 This will give you 4 setting triangles with the grain line running on the outside edge of your top.

For corner triangles:

Take your finished size / 1.414 + .875 for seam allowance. 

So, yours would be (15 / 1.414) + .875 = 11.48.  Cut a square 11.5 inches and slice on one diagonal.

This will give you 2 corner triangles with the grain line running on the outer edges of your top.