I appreciate that you are trying to make sure that good information is being shared. I'll take any constructive criticism, because I try to be very careful to not spread misinformation and want to know when I'm wrong. That's why I did my homework before making that post.
You are sort of correct. What you are referring to is the dimension of the gauges. Manufacturers typically make the GO gauge the same length as SAAMI's minimum, and the NO-GO .004" longer than the minimum. That doesn't mean that .004 is the SAAMI tolerance, it just means that it's what the gauge manufacturer chose in order to keep customers well away from the danger zone and keep themselves out of court.
Check out SAAMI's official publication on chamber dimensions
here. In the chamber drawings, you are looking for an x inside of a circle next to a dimension. The legend says that this denotes a headspace dimention. These are the cartridges I checked:
7.62x39
6.5 Creedmoor
223 Remington
308 Winchester
6.8mm Remington SPC
270 Winchester
30-06 Springfield
Every one of those has a tolerance of exactly .010".
I'll admit, I didn't mic a piece of tape right before posting that, but I have before and memory told me it was about .002" (but this will vary by brand). So I did a little bit of google-fu and came up with a range of .0015-.003, and posted "about .003" to err on the side of safety. If someone puts tape on their gauge expecting it to be .003" and it's .0015", then that means they are .0015" farther from having too much headspace.
Again, I appreciate you trying to make sure my information is accurate, and I'll do the same for you and hopefully we'll both be better for it. Pleased to make your acquaintance bohem!