When I do HST's I add a full inch to the FINISHED size of square that I want (i.e., 3.5" finished HST block gets CUT at 4.5"). I lay the two squares right sides together, draw a diagonal line down the center of the square and sew 1/4" on either side. Cut; press. Yes, I have to trim them down but whether it's my sewing that is not accurate or my initial cutting that is not accurate, I get a very accurate HST with this method. I've never had luck with Thangles and haven't tried the other methods that you mention. I place my 45 degree line on the sewn center line of the HST and trim to the correct size from that measurement. It's a very tedious process to be sure but it's the one that works best for me. But it was a good excuse to buy a rotating cutting mat!