/ic/ - Artwork/Critique

I'm doing Bargue plates, and I'm not sure exactly where my focus should be. Each plate takes a little over one hour to do. If I could, I want to get to a point where I can accurately sight check stuff without autistically measuring every distance. Is this a realistic goal?
Can you get sufficiently good at sighting to be able to capture a likeness, or will you always have to measure?
Or is it not even required to be perfectly accurate to capture a likeness? What does an acceptable error margin look like?
Should I check stuff as I'm working, or keep working until something pops up as being wrong? I only care about the method that gives me the most experience.