Luxire has preset size chart for their regular slim and extra slim fit shirting. I know every one thinks they are special and hard to fit. Humans are built in the generally to the same ratios. Check your measurements against the "standard". If you are wildly off your shirt will most likely be F-ed. Small tweaks should be made based on preference (otherwise why do mtm). In summary use the standard listed sizing as your sanity check. Check the temple of jaws thread. Drew (for the most part) is moving away from m2m because his standard sizing should work for 95% of the population. I bet the measurements for that Ill fitting shirt shown are way different then the standard measurement for the neck size he ordered.
Yes. I arrived at my measurements be tweaking from an existing shirt and going through a couple of iterations of changes. Lo and behold, I check them against the measurements of the extra slim fit in my neck size and they're very, very similar.