There are 2 basic shapes of snail shells: tall shells and wide shells. It is easier to measure the height (length) of a tall shell than it is to measure its width (diameter). The reverse holds for wide shells. In other words, because of their uneven shapes, the longest dimension of a shell is always easier to measure than the shorter ones.
The "standard" diameter of a tall shell is measured perpendicular to the coiling axis, also known as the columella (the white line in the picture) of the shell.
But this requires that the columella be held parallel to the caliper jaws. Easier said than done. To take the above picture, I had to support the shell with plasticine.
Some years ago, I realized that it was much easier to measure the oblique diameter of a shell. Here is one way of measuring the oblique diameter*. Note that the columella is not parallel to the caliper jaws.
The 2 diameters are, of course, not equal to each other. The perpendicular diameter of the shell in the pictures was 5.8 mm, while its oblique diameter was 5.5 mm.
The shell in the pictures is Batillaria minima, the subject of 2 manuscripts I am currently writing.
*I showed another way of measuring the oblique diameter in this post. It is imperative that the measurement method be explained in a manuscript so as not to leave any uncertainty.