This is an introduction along with useful formulas and an example. It will be updated periodically with several different examples. Statements are generally excerpts taken from my workshops on this subject.
The formulas I currently use do not agree with all texts. Throughout history, several different formulas have been used for the same named Lot. I have seen at least five formulas for the Lot of Marriage. There are many reasons for this seeming confusion. Formulas have fallen into dispute, perhaps due to inaccurate translations, misunderstandings between astrologers and students, lost manuscripts, or idiosyncratic thinking on the part of a translator. Cultural changes have affected the understanding of the original meanings and logic behind some formulas. This is easy to do if one considers only the descriptive names of the Lots, and does not look behind that name to review the planets or cusps included in the basic formula. So far, I would say the formulas always make sense when the planetary meanings of the formula components are considered.
To select a correct formula, it is helpful to consider the following ideas. Do you understand the meaning of the particular Lot? Does its use yield accurate information when placed in the chart? Does it seem out of context or invalid? One of the Lots of Marriage is Ascendant + 7th House cusp - Venus. This Lot seems to describe one's feelings toward close relationships. Another formula is Ascendant + Venus - Saturn. This Lot may best describe the legal contract or commitment aspect of marriage rather than the feelings involved in the relationship. I have found it may be better thought of as a business type Lot, instead of our romantic view of a new marriage. It does seem to have 7th house connections, and Western astrology customarily views business contracts or partnerships as coming under the auspices of the 7th house. I use it for agreements requiring personal responsibility, not a marker for information on affectionate ties. The formula including Saturn definitely has been indicated in committed relationships (legal marriage); but to get an idea of the feelings involved, note the lot that includes the 7th house cusp as part of the formula. If you think about it, doesn't it make sense that the 7th cusp is involved also in any contract? Many astrologers have used the Asc + 7th - Venus formula for business contract information and found it works just fine!
Complicating this whole idea, the opposition of the Lot of Marriage, using the first formula, is the Lot of Divorce. Obviously, a square to one will also be a square to the other. Some sources say to reverse the formulas of the above Lots if the individual has Sun in the lower hemisphere, or below the horizon in the natal chart. Considering that, and the idea of applying squares indicating building, and waning squares indicating a tearing down or adjustment, there are two reasons for finding out through many experiments which method is the best choice. Clearly we need to investigate, standardize formulas, and integrate them into our basic teaching in order to systematically select the correct ones for any particular chart.