I've used Thangles to pp HST's and, honestly, am not a fan of those. Generally for HST's, I cut 1" larger than the finished size, pair 2 squares and sew on either side of the drawn diagonal line. Granted, I have to trim, but they are accurate after that. And I press those seams open as well. Then again, I press most seams open. I like the flatter block.