[Turing_Machine]

All you need to know there is that a 3 mil is thicker than a 2 mil. That doesn't require any kind of calculation at all.

[int_19h]

Most units are used in the same manner - no-one is mentally converting kilometers into meters to compare two km distances, for example.

The point is that people do have a need for units that are tiny fractions of an inch, even in day to day life.

[Turing_Machine]

Miles and km are the same, just on the the other end of the scale. A straight magnitude comparison requires no calculation.

The units that people need to perform calculations with are the ones used in measuring for cooking, sewing, home improvement projects, and things of that nature.