The math given is incorrect, but you should probably do as amma suggested and try it out with paper before you cut cloth. Whether you use call it a 6" square or an 8" square, 6.875" is not the proper measurement for either setting triangles or corner triangles. It would give you corner triangles for an 8.5" finished square.
Just make sure you are aware that doing it Patrice's way, no matter what the correct measurement is, with HSTs as the setting triangles, you will have bias edges all the way around except for the corner triangles which are supposed to be HSTs. Setting triangles are quarter square triangles.