I noted the levels needed to maximize intestinal calcium absorption because this is the best studied aspect of Vitamin D. Rickets was the main reason Vitamin D was discovered and supplemented.
Vitamin D level blood testing is imprecise - I would not consider any one test of Vitamin D levels to be accurate.
This person's cited paper corresponds with the calcium absorption data that suggests around 8,000 IU D3 is where calcium absorption begins to plateau and more vitamin D leads to minimal additional calcium absorption.
Since I haven't been going outside, I take vitamin D 10,000 IU daily. You are correct in that one shouldn't just take the advice of random internet strangers - I should be more careful in posting. My personal experience with Vitamin D has been extremely positive and so I get excited about sharing what I know - of course I had to work and couldn't actively respond to this thread as much as I would like.