I used to get crippling Seasonal Affective Disorder every year, got tested, was badly deficient, and now don’t anymore.
I take 5000 IU most days.
Unfortunately Dr Berg seems not to consider the concept of biological half-time. I am sure he know the concept, just forgot to put it into his calculation formula.
He basically assumes that if his patient consumes 2000 IU vitamin D today, then tomorrow his patient has depleted all 2000 IU and has zero vitamin D in patient's blood.
This is obviously wrong.
Here's one example of how incompetent he is: https://www.youtube.com/watch?v=JvKNzLRmzLg
He basically turns low quality listicle nutrition pointers into videos with a whiteboard.