I confess that I don't really waste much time with carboy volume determinations.
If ever I do, I'd probably use the laundry marker method.
Yet, the labels look really nice.
Given that glass carboys come in all shapes and sizes (no standarization)--don't know how one could guarantee the accuracy of the labels as the vertical height of each gallon in one carboy won't necessarily equal the vertical height of each gallon in another carboy.
Better bottles, on the other hand, are standardized.
I don't know why they don't just have ridges already on them (i.e., created during the forming process).