Biblical generation length

In Genesis 5 we see before the flood that the average age of when a couple had their firstborn was about 100; for the men. Then in Genesis 11 after the flood we see it drop to about 30.

Now in Gen. 15:16 Father tells #abraham that his descendants will return to Canaan in the 4th generation. I take it that the generations are counted from Jacob entering Egypt. So the 4 generations are then Levi, Kohath, Amram and then Moses.

That then is more like 50 years per #generation if we use the calculations in the video on

If however we use the traditional view of 400 years in #egypt then a generation would be 100 years.

Do we keep the average age of 30 (as in Gen. 11) as the length for a generation throughout the rest of the Bible or do we adjust it according to the passage we read?