You can use 3ml or even 2ml for a 30mg per ml strength. Probably can't go much beyond three for the 60mg vial. Another option is splitting the 60mg vial across two bottles and freezing half so it doesn't sit around forever when you're only using 2.5mg and slowly increasing.
Restarting after several months off and now have 60mg vials instead of the 30mg ones I used before when I last stopped at 7.5mg. The BAC water amount is flexible. Using the calculator, 3ml in the 60mg vial means a 13-unit draw gives 2.5mg. Main worries are the lowest BAC volume that still works for 60mg and whether the vial can physically hold it since I don't have an old one to check.