That seems a bit loose....Ussher had a different age of Adams creation.
Why was his date wrong and the date you accept right?
Because Ussher used only 1,000 years for an age/generation, whereas in the Book of Revelation and Daniel, an age was 1,000 years plus the Little While Period which seems to fall out as exactly 24 years. Also, Ussher also used that a Prophetic Year consisted of only 360 days and then converted that to solar years and if you used his formula you arrived at his same understanding. However, in Daniel 9:24a was given 490 years of the Sin Sacrifice to repent of their idolatrous worship during the first two ages of the existence of the nation of Israel from the time of Isaac's birth. However, Ussher used his formula of multiplying the number of years by 360 and then dividing that number of "days" by 365 to arrive at the number of solar years which then changes the number of applicable Sin Sacrifices from 490 opportunities down to 483 opportunities for Israel to repent.
Now if we consider the Chronology of the OT Patriarchs from Adam to Jacob, we will discover that Isaac was born in the year 2,049 from the creation of Adam and not in the year 2,048th year from Adam's creation, accord to the
Seder Olam Rabbah, which is also known as the
Book of the Order of the world.
Now the OT record indicates that Terah, Abraham's father died one year before Sahar died, but because Stephen said in his defence before he was stoned to death, that Abraham left Haran after his "father" had died and as such because some scholars believe that Stephen was referring to Abraham's natural father and not to his Historical father, Noah, who was the only Patriarchal Father who died during the time, possibly around 25 years, that Abraham lived in Haran.
I would humbly suggest that my understanding is not loose as you have suggested, but that Ussher fudged his OT chronology to come to what he finally published.
Now if Isaac was born at the beginning of the third age of mankind, then there were 2,048 years in the first two ages such that an age therefore has a duration of 1,024 years which then means that the Little While Period that will exist after Satan is released, after he is imprisoned, must only have a duration of only 24 years if he was judged at either, the very end of the sixth age, or at the start of the seventh age, of mankind. The maths is consistent in either case.
Shalom