From what I know there's 2+ ion transporters in the intestines which transport Ca, Mg, Zn and all of the other 2+ ions in the diet.
The body in general will take what it needs from diet. Consider if you were in the sun all day like you would be previously if you were someone who worked outside all day. You ate real foods. I think the body is smart enough in general to absorb what it needs whne you;'re getting at least 10k IUs a day in vitamin D.
If you're worried I would just take a bit more from than usually. Since it's competitive inhibition more substrate in there will "force" more into the body until your body doesn't want it anymore.