I have based my calculation on the Jewish calendar, particular the Jewish Era. So today (year 2007), the Jewish Era would be the year 5767; in our time of 2000, it would be the year 5760 of the Jewish Era. This Jewish Era indicated that the Creation of the World and the Creation of Adam was 5767 years ago, ie 3767 BCE.
From the Jewish Era, I can work out when each patriarch was born and died, from Adam to Jacob, using the Genesis. That's how I calculated the
timeline of the patriarchs, which you can find on my website
Dark Mirrors of Heaven.
This how I calculated when the Flood happened, 2104 BCE, which is 1656 years after Adam's creation (Jewish Era or JE). Noah himself was born 1056 years after Adam's Creation (JE), thus he was born on 2704 BC. Since Noah was 502 when Shem was born (thus 1558 Jewish Era or 2202 BCE), 2 years after God's warning and instruction on building the Ark. Thus Shem was 98 at the time of Flood and 2 years later Arpachshad was born. So if Shem lived another 500 years as stated in 11:11, then his death would happen 2158 year in the Jewish calendar, ie 1602 BCE.
My calculation of Jacob's birth put him in 2108 Jewish Era, which is 1652 BCE, thus Shem's death happened after 50 years after Jacob's birth.
It stated that Sarah was 10 years younger than Abraham (born in 1948 JE or 1812 BCe), so she would have to born on 1958 JE or 1802 BCE. So if she died at age 127, it would be then 2085 JE or 1675 BCE, which is 23 years before Jacob was born.
Leaving out the BCE or JE (Jewish Era) for the moment, I think you did some miscalculation between Shem's birth to Jacob's birth.