If you clean them by boiling, or some other method and completely dry them. Then pack them by spinning, or letting them sit and continue topping them off then.
1 cart = 1mg
If you just refill them using Carto Cap Method with no spinning then:
1 cart = .75mg or about 40 carts for a 30mg bottle.
By the way, 33 drops = 1mg (assuming a drop size from a 5ml bottle) I have not checked the drops vs. ml with the 30ml bottles. The drop may be a different size.
Disclaimer: I have verified the numbers above a number of times. Your results may vary.