@kayskid - While I was on the Big 3, my ID doc recommended 5000 iu per day of Vitamin D3 as I was also very low. Now that I am off antibiotics, I am waiting to have my levels checked with my next labs.
Like Don, I think I get plenty without the supplement by spending hours every day outdoors (well, maybe not in winter if we have to stay in the frozen north this year.) For anyone taking calcium supplements, adequate Vitamin D levels are crucial to being able to absorb it.
Sue

Here ya go:
The recommended daily amount of vitamin D is 400 international units (IU) for children up to age 12 months, 600 IU for ages 1 to 70 years, and 800 IU for people over 70 years.Oct 18, 2017
Mayo Clinic › art-20363792
Vitamin D - Mayo Clinic