From my personal figures and research
If you live in the southern states of Aus you do need more
When in Vic I took 4000 per day to keep my D levels up and that included daily 2 Km walks on the beach but mostly well covered
Now back in Qld where we get 12 hours of sun a day and blinds are hardly ever shut and clothing is shorts and tee shirts all the time, I take 2000 most days to keep the same level of cover (never asked), just the GP comments that it is suitable
But facts are that if you allow your levels to get dangerously low supplements do not help
You need the sunshine and then use compliments to adjust to a suitable level which I have to ask the GP to test to arrive at the figure
Vit C, I generally know when it is high or low, due to hair and nail growth and the health of those items
Vit D3 needs a test done which all should get done, IMHO twice per year
