I am not exactly sure what was being asked in the opening post, so make this reply more out of one particular manner of reading the question in terms of numbering.
Many of us often use an Atout's (a Major Arcana's) usual numbering as a bit of a short-cut to writing the card's usual title. Hence, writing of the Chariot, I may instead use 'VII'.
This, for me at any case, calls to mind the whole card, rather than focus on what may otherwise be either only part of the imagery. For example, I would not usually only focus on the chariot itself when discussing the card, for what of the charioteer, or the horses, or the 'wand' the former holds, or the epaulettes, etc?
There are problems with this, of course, for not all decks either number or name the cards in the same manner.
Still, for myself at any rate, I tend to use the additive form of Roman as an abbreviation for 'standard' Marseille-style numbering.
When it then comes to also discussing numerological aspects of the card, I personally tend to principally apply geometrical aspects that may be considered, but do not usually 'reduce' a number such as XVIIII by first writing it in our common hindo-arabic form (19) and then adding the digits as though they were separate numbers (1+9 = 10 = 1+0 = 1).
Though there is much value in also working with such a method with regards to numerology generally, I personally prefer, when working with Tarot, to first and foremost focus on the image, its ordinal position (ie, what comes before and after), and its possible other relation to other cards within the deck (perhaps even suggested by pairings using the additive roman form of its number - again for example, XVIIII with VIIII).
