I know this is a little OT but since Part of Fortune came up I wonder if you use a different formula for day births and night births as Robert Hand suggests in
http://www.astro.com/astrologie/in_fortune_g.htm
In the article Hand says all the Lots have different formulas for diurnal and nocturnal charts.
I will probably get in trouble for replying to such an off topic point so fully but I think you have raised an interesting point.
Yes I do use the reversed formula for the Lot of Fortune/Spirit. Most authorities on the lots use the reversed formula ie Robert Hand, Dorian Greenbaum, Chris Brennan, Robert Schmidt and Steven Birchfield do this. Anyone strongly influenced by mainstream classical astrology is likely to do this. There is no question that reversal was the approach of most astrologers in the classical period with the Lot of Fortune. Ptolemy's rather maverick approach to the lots ie leaving out the lot of Spirit became more influential on medieval and renaissance astrologers due to revival of interest in his ideas. In particular the availability of medieval translations of Ptolemy's Tetrabiblos and Almagest.
The basic logic of reversing the lots relates back to planetary sect. In ancient astrology it was very important whether a chart was a day or night chart. In a day chart the Sun was sect light and of primary importance while in a night chart the Moon was sect light and of greater significance. Hence in a day chart the Lot of Fortune is calculated from the position of the Sun to the Moon but in a night chart the Lot of Fortune is calculated from from the position of the Moon to the Sun. In each case these are then projected from the ascendant. The Lot of Spirit is calculated in the opposite way ie in a day chart from the position of the Moon to the Sun and visa versa at night.
Ptolemy, unlike his contemporaries, did not reverse the calculation of the Lot of Fortune in day or night chart. Moreover, he did not use the Lot of Spirit at all. Ptolemy's idiosyncratic stance on the lots was very influential in the long term and became accepted practice for much of medieval, renaissance and early modern astrology.
Another factor leading to the decline of lot reversal was the changing understanding of what planetary sect meant in the horoscope. The fundamental distinction between a day and night chart became altered in Arabic and later medieval astrology. Hence the logic of Lot reversal was significantly diminished.
Personally, I think leaving out Lot reversal for the Lot of Fortune and Spirit lacks the philosophical consistency of the original approach. As I see it the relationship between the Lot of Fortune and Spirit tie integrally into the the lunation cycle and the ideas of planetary sect which are at the heart of ancient astrology. However, as in every debate there are always two sides of the argument.
By the way, I recommend Robert Hand's excellent booklet on planetary sect 'Night and Day' which you can order from Arhat publications.
Mark