The dimension measurements, e.g. evaluations of critical dimension (CD) and d spacings, in TEM cannot be accurate (normally 10% ~ 100% inaccuracy) if the camera length is in error. To ensure that the installation-calibrated camera length can be used in your experimental measurement, the specimen should be positioned in the "Eucentric plane". This can be done simply by manually tilting the specimen and thus finding the height of the specimen holder where the image of the specimen remains stationary.
On the other hand, the magnification of the image-forming lens system has normally an error of 5% - 10% so that the final magnification has an error of 5%-10%. To obtain more accurate magnification (and thus dimension measurement), the magnification should be calibrated with a lattice image of a known standard specimen by recording both the specimen and standard in the same field of view. Furthermore, the instruments may give a few percent of image distortion which also needs to be calibrated.
In some TEMs, the magnification and its aspect ratio of TEM Images can be automatically corrected in some degree even though it is not "perfect".