I have used exact grid methods using paper grids and that would solve your problem. But I really dont like this way.
My prefered method is El Burns, using large squares and drawing an x then sew 1/4 inch on each side. Cut in half both ways and on drawn lines to get 8 HST. They are slightly oversized so need to square up with her ruler that only requires one cut.
It is fast and they are perfect every time. For 2 1/2 inch squares use two 6 inch squares. For 3 1/2 use 8 in squares.
I love making HST now.