You could always go back to the very old fashioned way, cut templates, trace and sew. If it works, you can measure and do it the same size with a rotary cutter and ruler. If it does not work the way you want it, trim or make a new template. To make a template, take a piece of cardboard,(cereal box) draw the size of the finished square and draw a diagonal. Cut the cardboard, add a quarter inch all the1970 way around or trace onto the fabric as is and cut a quarter inch away from your tracing. I know this seems like a lot of work, but you have already spent time and it has not worked so I think it is worth it.
I taught quilting in about 1970 before rotary cutters and self healing mats. Everyone just cut and traced. The tracing line became your sewing line. Good luck.