Hi new to the list.
when searching the list for alignment tools I have found the "carpenter square plum-level" in the files section but not the "mirror" version.
The mirror version is good down to 0.1mm/m without much work,
basis is that we can likely agree that a long plumline if reasonably well sheltered from wind will be aligned to local gravity.
If a mirror is set so it reflects line near "top" and placed so it allow one to see the line near bottom even minute difference in alignment show up.
How accurate is this method?
If you think of how a vernier calliper works by allowing you to see minute "kinks" in the straight line you can readily understand that this would allow you to see deflection of under 1/4 of line diam easily, likely even 1/10.
if you have a 2m long line say 0.5mm thick (fishing-line, copper wire scavenged from old motor or transformer...) you would be able to see a deflection of say 0.25x0.5mm over 2m which turns out to be just under 0.07mm/m, not too far from many machinist levels.
materials needed, a strip of glass about 15-50mm wide 4mm thick (or more) long enough to rest on both pipes of the ways a thin line and a weight.
same strip of glass can be used to check straight-edges.
another use is as miniature surface plate for scraping purposes...