On 30/06/2012 11:49, F Murtz wrote:

If the vertical has only one bubble unit it may or may not read upside
down.


Use the other edge. You have no reason to believe that the two bubbles
will show the same level.

Come to that, I don't think I've ever seen two for opposite levels. Mine
all have a vertical and a horizontal bubble, and a couple have a 45
degree one.

Andy