Regarding measuring rods for outside micrometers, my questions are:
(1) to what degree do you trust import measuring rods?,
2) do you zero your micrometer before using it?, and
(3) should I be concerned about buying used measuring rods?
For my imperial mikes, I have Shars standards and Shars gage blocks.
For my metric mikes, I bought used Starrett and Mitutoyo standards.
As far as what tolerance I am shooting for, it doesn’t actually matter, BUT I do take my hobby seriously and I shoot for .001” or better. I fail more often than I succeed. I am striving to be able to work to .0003” one day; like I said, it will probably never have any practical application because most of what I do is just an exercise.
I make no effort for temperature control, but I do make an effort to clean the surfaces of oil before I take measurements. That should give you an idea of what precision I am aiming for. For myself, I consider anything much over a thou out to be a failure (or should I say a learning experience?).