The Prospec is really the key. It gives you key points in the right places, e.g. eyes. The rest is easy. Try it! As you start with that, you'll use it less and less; it trains the eye. In my case it helped to always use the same computer screen and the same paper size. Scale becomes automatic that way.
PS: I do crop and resize all ref photos to 1920 x 1440 (4:3) in software, to enforce equal display scale every time.
Yeah being consistent makes sense and not needed the additional tool after a while. I'm impressed by the drawings though. They're very realistic.