You are making it harder on yourself than you need to (we so often do that! LOL). Measure the current dark piece and add 1/4" to 1 side (the side already sewn on). Sew it on, press, then measure the side and add 1/4" to the side sewn on. That will be the measurement for the side pieces. Yes, you can always cut them a tad bigger to square off later, but these measurements should work. Another tip for all of us who can be math challenged at times is to trace the block onto graph paper, cut the pieces out (after labeling them) and measure adding 1 /4" to all sides (except outer side if not attached to anything because that 1/4" seam allowance is still there).