NavList:

usual "precision" levels bought in hardwarestores/buildingsuplies... are usually 0.5mm/m, those cost something in the region of 10-50Eur.

0.5mm/m turns in to roughly 1.8 arc minutes, with that and care you can get within 1 arc minute. should even be possible to correct for the inevitable tilt mathematically -if you take enough sights across large enough part of the sky- and precision of that correction will increase with number of sights.

as you likely have discovered leveling a mirror like this is NOT quickly done, it involves carefully adjusting until bubble appears centered then turning the level end over end along a line (or even better straight-edge glued to glas) and repeating until no movement can be seen.
Just for fun I will try tonight with a first surface mirror "rescued" from a transparancy projector to see what kind of result i get (I have several 0.5mm/m vials and...)

Ps:
here is a site showing sensitivity in several ways, they also sell vials and levels. forgot who posted this link first, sorry, whoever it was.
http://www.leveldevelopments.com/sensitivity-explained.htm have a nice table
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