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In reference to making the water level tool work just a teensy bit better:

First, when the water is almost at the same level at both ends of
the tube, the differential pressure is almost nil - so any resistance
to free flow will leave a residual error.

One of the major sources of such resistance is surface tension at
the air/water/tubing ID interface (with some plastics that the tubing
is made of, being far worse than others). This effect can be significantly
reduced by mixing a small amount of detergent (liquid dishwashing
type works fine) into the water before pouring it into the tube. I've
also found that adding a bit of food dye to the water as well makes
it much easier to read.

Also, if you remember your high-school chemistry lab work, the surface
of the water inside the tube will form a curve with the low part in
the middle. (This curved surface inside the tube is called the meniscus
-in case you're interested) and you should use the bottom of the meniscus
as your measuring level at each end - it provides the most accuracy.

Lastly, get ALL the bubbles out.

By the way, I read somewhere that the ancient Egyptians used a similar
method to level the foundations of the Pyrimids, so there really isn't
anything new under the sun - at least in this century.

Cheers,

Dan Schaefer
N235SP