To my understanding Vitamin D is regularly underdosed. Several points:

1) There are lots of studies that correlate Vitamin D production with sunlight exposure. For example, https://pubmed.ncbi.nlm.nih.gov/20398766/ this one lands on 1/4 of a MED = 1000 IU. Of course now we have a MED definition problem, but we're roughly talking single digit numbers for a white person in midday sun in NYC to reach 1/4 of a MED.

2) If you also supplement with Magnesium, a lot of your side effects go away. Vitamin D3 depletes Magnesium absorption.