In a level, both circular and tubular, the sensitivity is given by the bending radius: longer is the radius, more sensitive is the level. Usually in tubular glass levels, the sensitivity is indicated by an angular value for a physical 2 mm. movement of the air bubble, that is the distance between the notches of graduation (e.g. 20” x 2 mm.) or also specifying the thickness at the end of a 1 meter long hypothetical rod (e.g. 0,1x2x1000 mm).
If we have to establish which sensitivity we need in a level, we can use two trigonometries.
Giving the thickness (S), it is: S/1000 = arctangent of α = degrees. Vice versa, giving the angle in sexagesimal degrees (α), the thickness (S) can be found with Tangent of α x 1000 = S.