Measuring Guide ... The Height
Wherever possible measure the overall height of each wall, ideally in three positions along the length of the wall, i.e. the left corner, the centre point , and the right corner. (Do not be surprised if these measurements vary by as much as 10mm)
Once you have taken these measurements note the "smallest" length on to your plan outline, example below:

Why the smallest measurement? ... The illustration above shows the the ceiling is not level (it is lower in the left hand corner).
If you had only taken one ceiling height measurement (in the middle of the room where it is easiest to take) you would not have discovered a potential problem until you started fitting the kitchen.
For example a 2425mm high larder unit will fit in the right hand corner, but the same unit will not fit in the left corner.
If the smallest measurement is always used this situation will not happen.
All done? - lets move on to measuring ... the openings
Measuring ... | Outline | Width | Height | Opening | Evil Items |