Rulers should be accurate to within a small fraction of a millimeter over the entire length 0-22 cm.  Measure using an accurate ruler and if inaccurate email the instructor with the error, in millimeters, between the cm lines over the entire 22 cm length.

Note that the actual milllimeter marks might not be positioned with complete accuracy.  This is due to the impossibility of correctly placing pixels; this inaccuracy should be corrected soon.  However in the meantime note that this will introduce a small additional uncertainty in your measurements; do your best to compensate for this uncertainty.