Who care if your 1/2 in chisel made overseas is an reality a metric chisel labelled somewhat close to 1/2 in, but not really... Uses whatever you have to your advantage.
If I need an 1/2 in mortise, I will set my mortise gauge using my so called 1/2 in chisel, that it is a smidgen wider or smaller, I could not care less. And I will then make my tenon to fit my somewhat 1/2 in mortise by sneaking up to the fit.
What about digital measuring gismos, aren't they inherently more accurate? Not really, they will give you a close approximation, never an absolute, and probably never twice the same exact measurement yet within the stated accuracy.
You see, the problem with measuring devices is always in the interpretation or interpolation, both from a human or some sort of electronic transducer. Real life examples rarely fall exactly on the tick marks, more as in somewhat in between... as in 2 little bar and a smidge past the big number 5. Read it again but tilt your head slightly the other way and it is now more like 1 little bar past whatever. Parallax errors anyone? Even when you are aware and compensate for it, no two people are going to measure and interpolate the same.
What about converting back and forth between measuring systems such as Metric and Imperial? Now you are asking for troubles, rockets blew up on the launching pad and space exploration probes have missed the mark completely because of it! Stick to whatever you are familiar with.
What if you were to take your piece and mark it where it will fit? Ah, but what about the thickness of your line? How about sneaking up to the exact length...
Have you ever tried to shave off a smidgen with a power miter saw? Good luck and watch out, it could get seriously dangerous in a hurry. On the other hand using a shooting board and a sharp plane it is very easy and safe to do.
Need to remove a few thous from a board thickness? If your plane is set to remove a know thickness per pass it is easy to do. One caveat here, trying to measure a fluffy shaving with a micrometer can quickly becomes an exercise in frustration. The pressure you exert with your micrometer or caliper will affect its reading. So don't get worked up about it. and give me an analog device over a digital one any day!
Ever tried to read off a measuring tape to check if your assembly is square during a glue up?
Heck of a lot easier using pinch rods, or gauging sticks and more accurate too (Hint it is because there are no actual measurements involved).
Ever used a story sick or pole? Once you understand how it is used, you will be amazed at the number of information it can carry, and accurately too.
Another source of errors is by using various instruments. Stick to monogamy here, uses the same measuring tape, rulers, try square whatever you are using. Every time you switch instrument you introduce yet more variations.
Setting your tools, calipers, marking gauges etc. using known dimension; Setting blocks, specialized gauges, are a lot easier than trying to lift measurements of a ruler for example
Learning to use and trust your senses, it is amazing how much accuracy we have "built-in" at our disposition. You can resolve a few thous difference with your fingers (you still have ten, right ? :-) or by seeing light between a know flat surface and an object. Starting to see the light yet?
And talking of senses, if your nose is smelling burning and your ears are hearing screaming noises from your planer or drum sander, you are probably taking too much of a cut or are experiencing bind up, pay attention to your senses!
Or I suppose, I should say "You should come to your senses!"
Bob, staunchly refusing to convert to Metric, long live my foot :-)