I've been watching this thread with great interest since - as I've already confessed - triangles and points are my biggest challenge.
I wonder ... might it not work better to draw the diagonal lines on the back of the "under-patch", then attach a slightly oversized square, and sew along the line on the back the same way you would if paper piecing? Seems to me it would be more likely the triange ends up in the right place and folds back to guarantee it could be trimmed to fit and line up properly.
Has anybody tried it that way? Does it work?