Choose the reconstitution volume so that your ordinary dose lands on a mark you can actually read. A dose that works out to 2.3 units is a dose you will measure badly every single time. If the division keeps producing awkward fractions of a unit, add more water and redo it. More water is not weaker medicine, it is the same mass spread over a volume your eyes can resolve. The trade-off is only how long the mixed vial lasts and whether the vial has room.
