Try this Aggi.
If you are sewing the bias edge of the QST (a short side) to the inner square, the length of the side of the square should be equal to the length of the triangle edge minus .875 plus .5
If you are sewing the straight of grain edge of the QST (the long side) to the inner square, the length of the side of the square should be equal to the length of the triangle edge minus 1.25 plus .5
You take the triangle side length, subtract the triangle seam allowances, and add the square seam allowances. Make sense? The only difference is the angles of the triangle that hit the edge of the square. (there's .25 for a 90º angle and .625 for a 45º angle seam allowances)
I get 10.94 for the side of the center square with the bias edge option which is confirmed by working backwards through the usual formula for setting triangles (Finished Block Size x 1.414 = Finished Diagonal + 1.25 = Size of Square to Cut). Don't know how you get 15.875 when the edge of your QST is only 11.3137...unless I totally misread something along the line. :oops: