I’ve touched on this subject before but wanted to spend some time going over the details. There are situations (particularly the “quintessence” calculation) where we add together the numerical values of all the cards in a spread to arrive at a single trump card that can provide a summary “roll-up” of the combined energies at work in the population of cards drawn. Obviously, with larger spreads, this total can exceed the number of the highest trump card, the World. The customary approach to dealing with this is to simply add the digits of the total together as many times as necessary to obtain a number below 22; this is known as “theosophical reduction.”
There are a couple of disadvantages to this approach. The biggest one is that this form of reduction can never give you zero as the Fool. Many people simply renumber the Fool as 22, which has an historical precedent in the work of French occultist Eliphas Levi. I choose not to do this. The second disadvantage is that this method can jump right over some of the trump cards in the sequence that might otherwise be a better fit for the context of the question. There is at least one reasonable solution to both of these problems. (There is a third disadvantage, which only occurs in small spreads of five or fewer “pip” cards, where the distribution of possible trump cards is mathematically skewed toward one end of the series, such that some may never be chosen by reduction; this potential doesn’t concern me here.)
The most effective alternative is known as “casting out nines,” in which you remove increments of nine from the total until you achieve a value less than 22. For example, 27 would give you the Hermit by theosophical reduction and you would stop there. However, by casting out nines you first come to the Moon, while removing a second nine gives you the Hermit and subtracting a third nine yields the Fool as zero. The significant advantage here is that you have choices as to which trump card offers the most relevant testimony, or you can read all of them as a composite perspective on the situation. The third option is to subtract 22 from any number above 21, which can get you to zero but doesn’t introduce as much flexibility as casting out nines. The latter, when I remember to use it, has become my preferred technique, and I will often interpret all of the cards singled out in this manner.
Here is a visual representation of the possibilities: