Yes, when you've done it a lot you get good at it, but I did it for 20 years, and was the best person at every shop I worked at frame alignment. But when I had a problem on my own bike with a 9-block triple setup I was not able to resolve a persistent shifting problem in some combos until I went to a shop and used their guage. As I stated before a guage is necessary to do an accurate job - the tolerances are much tighter with indexed high-count rear cog systems.

In the case where the eyeball method does not get it completely dialed one needs to informe the customer that there will be a separate charge for doing hanger alignment.