Straightness of a bed using a spirit level

Rise per division

Sensitivity $= 0.02$ mm per 1000 mm, so for the 200 mm base:

$$1\ \text{div} = 0.02 \times \frac{200}{1000} = 0.004\ \text{mm}$$

Heights

Each rise is $\text{reading} \times 0.004$ mm; the height at a position is the sum of the rises up to it.

The line joining the end points rises $0.028$ mm over $1200$ mm, so at the $k$th station its height is $0.028k/6$ mm.

Profile

 height (mm)
 0.028 |                       *
 0.024 |           *
 0.020 |       *       *   *
 0.016 |
 0.012 |
 0.008 |   *
 0.004 |
 0.000 *------------------------> mm
       0  200 400 600 800 1000 1200

The bed is highest above the end line at 400 mm ($+0.0107$ mm) and lowest below it at 1000 mm ($-0.0033$ mm). The straightness error is the width of the zone between two lines parallel to the end line that enclose the profile:

$$0.0107 - (-0.0033) = 0.014\ \text{mm over 1200 mm}$$

Answer: straightness error = 0.014 mm over 1200 mm (maximum hump of 0.0107 mm at 400 mm).