Every position of the Dragon Gate formula follows from the sitting palace by one rule, stated in the sources that print the tables: read the sitting trigram, and the facing trigram opposite it, against the Early Heaven and Later Heaven arrangements of the eight trigrams. Four printed tables agree with that rule on all 56 cells of the seven fixed positions.
The rule, position by position
先天: the palace where the sitting trigram stands in the Early Heaven arrangement. 後天: the palace of the trigram that stands, in the Early Heaven arrangement, on the sitting trigram’s Later Heaven seat. 賓 and 客: the same two for the facing trigram. 案劫: the facing palace itself. 天劫 and 地刑: the two palaces 45° either side of the facing — 天劫 on the left looking out for houses sitting in 兌, 乾, 坎 or 艮, on the right for houses sitting in 震, 巽, 離 or 坤. 輔卦: whatever palace is left; where none is left (the 艮 and 震 houses), 輔卦 doubles 地刑, and where two are left (the 坤 and 兌 houses), it holds both.
The eight formations
Sitting 坎 (N): 先天 W, 後天 SW, 賓 E, 客 NW, 天劫 SE, 地刑 SW, 案劫 S, 輔卦 NE. Sitting 艮 (NE): 先天 NW, 後天 E, 賓 N, 客 SE, 天劫 S, 地刑 W, 案劫 SW, 輔卦 W. Sitting 震 (E): 先天 NE, 後天 S, 賓 SE, 客 N, 天劫 NW, 地刑 SW, 案劫 W, 輔卦 SW. Sitting 巽 (SE): 先天 SW, 後天 W, 賓 S, 客 NE, 天劫 N, 地刑 W, 案劫 NW, 輔卦 E. Sitting 離 (S): 先天 E, 後天 NW, 賓 W, 客 SW, 天劫 NE, 地刑 NW, 案劫 N, 輔卦 SE. Sitting 坤 (SW): 先天 N, 後天 SE, 賓 NW, 客 E, 天劫 E, 地刑 N, 案劫 NE, 輔卦 S and W. Sitting 兌 (W): 先天 SE, 後天 N, 賓 NE, 客 S, 天劫 NE, 地刑 SE, 案劫 E, 輔卦 NW and SW. Sitting 乾 (NW): 先天 S, 後天 NE, 賓 SW, 客 W, 天劫 E, 地刑 S, 案劫 SE, 輔卦 N.
The water mouth and the 曜煞 mountains
The water mouth (水口) is where the formula wants water to drain away: the stem and corner mountains of the 天劫 and 案劫 palaces — for a house sitting in 坎, the 巽, 丙 and 丁 mountains. The 曜煞 are three mountains per formation where a road, a building corner or water striking toward the house is read as harmful: the 八煞 mountain of the sitting trigram, and those of its 先天 and 後天 palaces (the verse 坎龍坤兔震山猴，乾馬巽雞兌蛇頭，離豬艮虎). For the 坎 house they are 辰, 巳 and 卯. A sitting bearing within one line (0.94°) of the boundary between two palaces is flagged: across that line the house belongs to a different formation.
References
Canonical sources that inform this guide.
- 论龙门八局系水法 (华夏大易学社) · WEBSITE
- 三元水法龙门八大局 (简书) · WEBSITE
- Bagua (Eight Trigrams) · WIKIPEDIA
- Feng shui · WIKIPEDIA