My old prepress guy in Toledo swore by CMYK values from a swatch book, and now I trust my eyes more
Back in 2008 he told me to never trust a monitor and always match the Pantone chip, so I did that for years until a job came back with a green that looked nothing like the book. Turns out the press was running 8% heavier on cyan that whole week, and nobody caught it because we were all staring at the book instead of the sheet. Anyone else had a mentor's rule blow up on them like that?