the one third of 9.5 conundrum, which was posed by a metric defender
they weren't a metric defender
which is literally not true
what scenario is there in your mind where you'd need a precision answer to what 1/3 of 9.5mm is, but also not have access to a calculator? and of those scenarios, how many of them would be solved by the knowledge that 1/3 of 1/8 is 1/24? i'm willing to bet the answer is more or less "none".
and for those that do exist, you can also get drafting rulers that give you 1/3rds of metric measurements.
the accuracy of your equipment isn't somehow better because you're dealing with fractions rather than decimal points
using a ruler to measure a length of 1/8" is as accurate as using a ruler to measure a length of 31mm and eyeballing 2/3 of a mm
the bottleneck at that point is your eyeball and pencil lead, not the unit of measurement