NavList:

I suppose you know you can determine the sensitivity of the vial by stacking feeler gauges under one end to move the bubble one division, then do the trig to get the angle.

If you plan on placing this level on top of a mirror, I would suggest not using a flat base. The flat base will introduce errors sitting on the microscopic high points on the surface. I would suggest getting some 3/16" brass round stock. Cut two pieces about a half inch long each. Epoxy the brass rods at right angles to the long axis of the vial , one at each end. After curing,  you can easily  calibrate the vial , by testing on a very flat, fairly level surface . When you turn the vial 180 degrees, it should read the same in either direction. If it does not, you can easily run one rod across some fine sandpaper to lower one end to calibrate the vial. Draw the brass rod  across the sandpaper to produce a slight arc on the rod rather than a flat. I have played a lot with small levels. On my AH I use level vials from surveyers transits or levels with little brass feet.