Hey -- I found out later that calculator doesn't go up as big as your center.
At 35" (which may not be your correct final size) the diagonal measures 49.5 -- that would mean the corner triangles would be roughly 25" per side. To determine the size of a HST, we add 7/8th's seam allowance. For people like me, we round up that to a full inch (or more) bigger than our measurement.
Me, I'd cut squares about 26.5-27" and divide those in half. Sew on and trim to your finished square size.
Don't know if the diagram just makes it worse... but I'm trying to help
