Sorry I've to write in English, since it'll take me hours to type in Chinese...

I've got some primitive idea about "命宮"

Formula of calculating "命宮" 支 is

14 - (month no + hour no)

where 寅 = 1, 卯 = 2, etc
month no. corresponds to zodiac sign of SUN
i.e month no. 1 (寅) -> SUN in Aquarius
month no. 2 (卯) -> SUN in Pisces, etc
hour no. corresponds to SUN's House position
i.e birth in 卯時 -> SUN somewhere between House 1 & House 12
birth in 辰時 -> SUN somewhere between House 12 & House 11, etc

"命宮" is actually an approximate formula for sign of ascendant. Formula is not exact as birth time is not exact and birth location is not taken into consideration. But still it's valid for most cases, otherwise the actual sign of ascendant (in terms of 地支) will be one greater than no. calculated as in "八字".

Importance of ascendant (& it's sign) is well-known in astrology, so if "命宮" is applied more information will be disclosed than just using ordinary "八字四柱".

As revealed by the formula, "命宮" is more dependent on birth hour (birth hour being varying more rapidly than month no.), use of it gives another level of details (month 干支 gives some info, hour 干支 gives more info, and even more for "命宮"). Use of "命宮" will give more specification to the exact time of birth (actually more specific description of cosmic configuration in birth time), we all believe that more exact birth time is basis for more details fortune telling, so why same "八字" after 60-year interval doesn't mean same destiny.

Reason why "命宮" is often ignored may be due to the fact that we Chinese tends to abstraction to make things simpler (look at I-Ching), if "六字" will give enough information then "八字" will be avoided..

Since formula for "命宮" is not exact, you might have to add one by yourself in some cases (e.g 戊寅 replaced by 己卯), or better use astrolog to determine ascendant, and make the transformation to 干支. Well, it's only my thought, I haven't tried it myself.