You could achieve this without doing any math if you draw the block on paper in the size you want and then add the 1/4" seam around one of the triangles using a ruler. Now you can measure that and you know what size you need.
For ease in sewing the HST's you can measure the short sides of these triangles (including seam allowance) and cut squares that size, pin one light and one dark with right sides facing and mark the center diagonal, sew 1/4" on each side of the line, then cut apart on the line between the seams. I prefer this method because if you start with the larger square and cut both diagonals those are bias cuts, whereas the 2-at-time method has the bias stabilized with stitches before you cut, and the outer edges are straight grain. Also, they are easily chain pieced.