Getting everything square

[BillTodd]

A spirit level seems a bit lacking in accuracy

A good builders bubble level will be OK to a couple of mm over its length. A mechanist's level is capable of measuring to fractions of a mm. (e.g. <0.5mm/m) but at a price

You could probably make a precision level out of a good straight edge and a length of glass tube bent over a spacer in the middle of the tube (see attached). The tube has to be partially with water and a little glycol (antifreeze) to leave a small bubble. It doesn't have the internal barrel shape of a proper level bubble, but it'll still work.

Calibrate it by placing a a reasonable level surface, mark the bubble position, turn it around 180 degrees, mark the bubble position. Half way between the two marks will be level.

The flatter the bend in the tube the greater the accuracy and the smaller the bubble will have to be.