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Usually I work freehand, sometimes with a little help from a proportional divider. I made a post about this a while back:

https://peakd.com/art/@wulff-media/video-night-hunter-watercolor-painting-and-bonus-painting-hack

Once I'm clear where the lines are, I vigorously erase everything else, most notably with a little pencil eraser, and redraw what remains. Then comes the photo, and afterwards more erasure with a kneaded eraser to "bleach" the sketch. Naturally this requires good paper and soft erasers :-)

I see. That's interesting. It's really good for free hand, even with a proportional divider. Awesome!

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.