No, there was no guess work to it. It is based on careful scholarship, studying the genealogies and the exceptions to the genealogies that are in the Prophets, to cover the stated gaps in the genealogies.
It took Usher several years of work to develop his calculations.
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Are there not a few gaps in the timeline?
The Bible is, as far as I can tell by reading it in English, mute on the amount of time a) between placing Adam in the Garden of Eden and God creating Eve, and b) the amount of time between Eve being created and the incident with the tree of knowledge.
I confess my ignorance here on this point, so please, try to avoid making this about me and my ignorance (which on this point is freely admitted), and provide the information I lack instead.
Thanks E-S.
The “gaps” are called out in the prophets, when certain individuals were removed for their idolatry. It all works out quite well.
Adam’s and his descendants ages at the times their children were born are all called out in extreme detail, making computation of the length of the genealogies mostly a rote task, except for those whose names were left out deliberately, but covered in the prophets.
The New Testament genealogies finish off the job up to the birth of Yeshua, which is a known date. The differences in the two NT genealogies is because one is of Joseph (irrelevent) and the other is of Mary.