how small an angular change can a precision level register?

 TL/DR: minutes of angle for a "machinist" tool room grade level, and seconds of angle for a lab-grade level.

 After watching this video of Ca Lem making a very nice little precision level and testing it, I thought I'd try the same trick with my 6" L.S. Starrett level (a tool-room grade level).

 I obtained a single red hair from my wife's hair brush and mic'd it at .0025" in diameter.

I verified my surface plate is still level, then I jacked up one end of my level with that one red hair.  The bubble moved more than a full division off center.  Even a .001 feeler gauge made an obvious and readable difference.

According to Hipparchus, .0025 under one end of six inches equates to an angular difference of 0º, 1', 34".

Neat.