But, if the birangle is for 1x2 rectangles, then it would be for a 2x4" rectangle, not a 2x 3 1/2 one.
Graph paper your rectangle, draw an exact diagonal line, and cut it out of cardboard. tape it to the top of your ruler, diagonal side of the triangle against the cutting edge, then use your ruler.
7/8 still works for the 1x2 diagonals, but cut at 1" instead, just to be safe.
a HST angle is 45*
a 1x2 trianlgle is 22 1/2*
I drew a 22.5* angle line on my ruler with a sharpie.