Your videos are excellent, so clearly explained and filmed, easy to follow. So many techniques here I have struggled with. thank you. One thing we in the UK have to be aware of, is that what you call doubles, we call trebles !!
My only difference in doing this triangle, is that I choose to anchor that end treble stitch in the 4th chain stitch itself, instead of working in the space. That way the stitches don't slide.