I intend to print out an image of this standard when I print images of samples, in order to determine final magnification. Also there is no tolerance stated for the separation between the lines.
Measurements are from centers of the lines.
They're not cheap. If you want to do your own certification, start with some regular fine grid, perhaps a dime store holograph, measure it against some standard you trust, and figure out the spacing. For some uses it could be most useful to then establish the length of the negative in micrometers and use that as a standard, but some people would prefer large latex spheres incorporated with the specimens.