1. I am not sure if the YEAR formula for calculating Kua number is for the convenience for the Westerners who do not use lunar/Chinese calendar (fixed Feb 4 starts a new year for most years), or original FS has already used this almost-fixed formula on its own, having the Gregorian calendar in mind as a combination.
2. I did not care about clarifying this question, because neither my husband or I was born in Gregorian Calendar of January or February. However, my mother-in-law knows that I am leaning FS and is interested in knowing what kua number she is, so she can position her bed right. Her LUNAR calendar birthday is January 4th, 1950. Feb. 20 is the date I converted the Chinese Calendar online to get her Gregorian birthday. No discrepancy for her Chinese/Gregorian dates. However, what if someone wants to calculate his/her kua number later and is born January 1st, 2012 in Chinese Calendar, which converts to Gregorian as January 23, 2012, would using Chinese/Lunar calendar as birth year of 2012 or FS formula of 2011 for his/her birth year??? (see.... The year 2012 is not starting until Feb 4 as a FS formula, yet in Chinese/lunar calendar is already starting the year 2012)
All you need to know is to forget one calendar and follow two types calendar.
Lunar Calendar- Forget it. It only use to calculate the moon rotating and in ancient time only farmer use to look into something.
Gregorian Calendar- Always using.
Solar Calendar- This is you mention FS formula. So only this calendar are using for Bazi and Personal Gua number.
In this case, your mum LUNAR birthday is 4th First Month(in Lunar there is no Jan or Feb.) 1950. In Gregorian is 20th Feb 1950. In this case, you mum is born before 4th Feb thich know as Li Chun立春 of the year 1950.
SO you mum must use 1949 to find out her Personal Life Gua instead of 1950. SO her Gua no is 9.