Well, just my opinion, but having just used my similar gauge both vertically and horizontally, I find I get more consistent readings in the horizontal position.
Zeroing the gauge looks odd, as you have to hang it near the ball to set the dial, but it does seem to work well in the flat position.
Partly (IMHO) because the ball finds the bottom of the tube naturally by gravity.

Dumb, but learning...Prof Em, BSc(ME), CAE (FYI)