Here are example images of a replica diffration grating (aka a waffle grid) that can be used to calibrate the magnifications for a camera like the MegaScan 794 on the JEOL JEM 1010. This sort of calibration is based on measuring distances between the known spacings between the lines shown in such images, and measuring the distance between multiple lines can reduce the uncertainty in the exact position of each line.
The Electron Microscopy Center (EMC) staff was able to calibrate the magnifications for this camera on the 1010 up to a magnification of 12000x using the waffle grid, but images at higher magnifications show less than a full grid square and can not be used for calibration. Magnifications lower than 100x do not show the waffle pattern clearly enough to use for calibration, though it would be possible to use the spacing between grid bars for that purpose at such low magnifications. It is also possible to use such a series of images to determine the changes in rotation as the magnification is changed.
NOTE: Even though the nominal magnifications of the mag mode and the low mag mode ranges overlap, these images clearly show that mag mode 1000x and 600x are significantly different from low mag mode 1000x and 600x.