I used the Masoretic text for Methuselah and Lamech because these were probably scribal errors (see link below).
I also checked my numbers after completing against others and I was 69 years off from Josephus for date of Adam and 60 years off for date of flood (see link below). I like my dates better. It has Methuselah dying year of flood.
It appears that the Jews altered the Hebrew text and took 650 years out from Shem’s descendants to Terah’s father Nahor. I read one theory as to why they might have done this. It was to discredit the book of Hebrews concerning Jesus as High Priest and receiving the priesthood after the order of Melchizedek. This was to make it look like Shem was Melchizedek and past the priesthood on to Levi through Abraham’s seed. They needed Shem to overlap with Abraham in order to do this. By doing so, they could make the case that Jesus was of the tribe of Judah and therefore could not be a high priest, and use Shem as an example of passing the priesthood on to Levi..
Another problem with the traditional Masoretic timeline, it has the Pyramid of Giza being built before the flood. It also only allows for 857 years from the time of the flood until the Exodus (1491 B.C.). Not enough years to procreate, for the Tower of Babel, people to migrate and to begin to build other cities. The timeline I did has about 1,447 years from the flood to the Exodus.
Written history only dates back to about the time of the flood (3000 B.C.).