The four transformations in one paragraph
Every Zi Wei chart derives four 'transformations' from a Heavenly Stem: Hua Lu (化禄, smooth gain), Hua Quan (化权, drive and control), Hua Ke (化科, reputation), and Hua Ji (化忌, attachment and obstruction). Each transformation attaches to one star, coloring that star's expression for the whole chart. Which stem you look up changes the answer.
Year stem vs month stem
The mainstream convention — used by nearly every free online calculator — looks up the four transformations from the Heavenly Stem of the birth year. Kwok Man-Ho, whose books introduced Zi Wei Dou Shu to English readers, taught looking them up from the stem of the birth month instead, arguing the month is the tighter time-slice of the season you were born into.
For seven of the ten stems the two methods agree. For three — Geng (庚), Ren (壬), and Wu (戊) — they disagree, and on Geng they swap Hua Ke and Hua Ji outright. If you were born in a Geng year, the two conventions can produce materially different readings.
A worked example
Born 15 April 1985: the year stem is Yi (乙), the month stem is Ji (己). Year-stem method gives Tian Ji Hua Lu, Tian Liang Hua Quan, Zi Wei Hua Ke, Tai Yin Hua Ji. Month-stem method gives Wu Qu Hua Lu, Tan Lang Hua Quan, Tian Liang Hua Ke, Wen Qu Hua Ji — a different chart emphasis entirely.
Our position
FateZiwei defaults to the mainstream year-stem convention (the 'Flying Star' table) and lets you switch: the San He table for the three disputed stems, or the full month-stem method. When two reputable traditions disagree, the honest product answer is to show both, clearly labeled — not to silently pick one.