Interesting thread and it looks like everything has been said but nobody brought up the fact that the PPG toners to mix the paint have been changed over the years. Metallics are made different, toners are thinner, some have been superceded, no lead content, all kinds of EPA and cost based changes so an exact match is still a process of experience and time even if the formula exists.
Just to throw this out there, but I worked in a shop that used PPG and we tinted every job that came through that first year I was there. After switching to Standox, over the next 4 years we tinted 4 jobs and every day I had to listen to the owner gripe that he wished he had stuck with PPG. He just could not reconcile the fact that it was sold in liters and not gallons, so he swore it was too expensive. Anybody ever made money on a redo? Me neither. My Charger will be Sikkens because I mixed up a couple of free gallons before we changed out the toner bank for this waterbased stuff! I have a survivor type 74 Charger in G8 Dark Sherwood Green and Sikkens was dead on. I blended the end of one fender and about 200 years of experience couldn't tell where I stopped. That's just one color though. And would everybody PLEASE include the paint type and formula for your car in the sale so that the poor sob that has to touch it up later has a starting point to work with! It would just make life easier for us that do this everyday.