When planets are “180º away” from the sun their phase is full. Just like the Moon has phases, all the planets also have phases it’s just that since they are so small you don’t notice it except as a vague brightening or dimming of the planet. When the planet is full, it is brightest and thus can fully shine whatever strength it has. When the planet is 0º from the Sun it is just like the new moon – invisible, and completely dark. In that condition, even if a planet has a high Six-Fold Strength it has not chance to shine and thus exhibit that strength.
Like planetary war, this is another factor that does not directly figure into the Six-Fold strength calculation. It is something you must consider on top of the Six-Fold strength. A planet with a lower strength, yet full phase (being 180º from the Sun) will be stronger than you expect on the basis of the Six-Fold Strength alone. Similarly when planets get close to the Sun they will operate with much lower strength than what their Six-Fold Strength suggests. This is particularly noticeable within about 10º of the Sun, and reaches it most dramatic effect at exact conjunction with the Sun. “Combustion” is a common term for the planetary phases occurring at this degree of combustion.
Venus and Mercury are important exceptions to the 180º rule, because they never get that far away from the Sun. All planets achieve their “full” phase when they are at their maximum distance from the Sun.
For the outer planets this translates nicely into 180º away from the Sun. But the inner planets, Venus and Mercury achieve their maximum distance from the Sun at 46º and 28º respectively.