Hi,
< The only source she had said to do 2-3 IU/ml>
You are right! If this is all the vet said , then these are confusing instructions.

Often a vet will sent you home with a liquid calcium and instructions as to how to dilute it.

Many vitamins are measured in IU ( international units)
and
one ml is the same as one cc ( you can get a needless syringe at the pet store that will help you measure in..... ml or cc)
It may be a syringe that will measure a total of 10ml or 10cc

Is your calcium supplement liquid? Does it have information on it regarding IU measurements?
Can you post a link to the Ca supplement you have?

The vet should have given you a ( needle-less) syringe to measure this specific recommendation of fluid i.e. one ml ( or one cc of fluid )
You would then need to know exactly how much of your calcium supplement equals the recommended "2-3 IUs"
Then dilute this 2-3 IU in 1 cc ( or one ml ) of fluid i.e. distilled water
or
6 IU: 2ml
12 IU :4ml etc

FYI 5ml ( or 5 cc) = 1 tespoon

You could call the vet for better instructions /help with exactly how much of your supplement
( specifically )is used to make the ratio of 2-3IU: 1 ml of fluid

Like Don said... " start dusting " everyday anyhow !