Barleywine
I'm glad someone brought this up, because it's something that I find interesting but have struggled to resolve to my own satisfaction.
I use a quintessence card sometimes...not routinely with every reading, but when I feel the need for more information to wrap up a spread that otherwise seems to lack a certain cohesion. I experimented briefly with subtracting the value of elementally conflicted cards (since I don't invert cards), and found that it did seem to add another layer of meaning to the reading.
But I eventually discarded the practice because it seemed to be a rather artificial idea imposed from the outside, so to speak, and not to have any basis in the numbering of the cards. Using reversed/ill-dignified cards to "drag" the quintessence back to a lower-numbered card would make sense to me if the lower-numbered cards were somehow weaker or more conflicted themselves than higher-numbered ones. As it stands, though, I just don't see a basis for this practice that's inherent to the cards themselves.
I'm the first to admit that I'm not as strong in numerology as I would like to be (or even basic math...I tend to break into a cold sweat when faced with the prospect of calculating the tip in a restaurant) so if I'm missing something blatantly obvious here -- or even something subtle and complex -- I would be very grateful if anyone could shed further light on this idea.
The main point in subtracting reversed values for the quint is that it lets me keep the Fool as "0" and not have to treat it as "22." If you just add and reduce positive numbers, the lowest value you can come up with is "1." You can also get a reversed (negative numbered) quint this way, which makes sense if you're already using reversals.
I also use the quint mostly for spreads that are inconclusive; I prefer them to clarifiers, base/shadow cards and other auxiliary cards because they are focused on the cards already on the table, no extra cards needed. For the same reason, I always include court cards as "11, 12, 13 and 14." If I'm doing a very small spread - 3 to 5 cards - I like to use the quint as a summary card.