Instead of PP, may I suggest this to you...just as an alternative.....when you mark those squares with that diagonal sew that line just RIGHT of that drawn line..that allows for fold...and also when you trim, just trim the back of that square, that is the piece of fab in the middle-leaving the original rectangle corner there along with the new corner you have made....that original rect. is your true seam guide when joining those FG. And remember that seam you have created is on the bias...so sew without pulling/tugging and press lightly! I never piece anything on full speed...my full speed is med slow...and, I think, more accurate ( most of the time) ....