Great tip and a nice demo. It reminds me a little of the sight-size method used for Barguge drawings - with these, most use a skewer or knitting needle to estimate angles, but this is more practical and easier to implement. A very helpful skill to develop.
And this technique doesn't just apply to when you use reference photos. When drawing from life I quite often "draw" in the air over the subject - looks a bit odd but helps a lot.
