Most manufacturers include 10-20% overfill to account for waste during reconstitution and drawing. Calculate concentration based on the labeled amount, not the overfill. For example, a vial labeled 5mg with 5.5mg actual content reconstituted in 1mL yields 5mg/mL working concentration, with the extra 0.5mg serving as buffer for measurement errors.
USP guidelines address overfill standards in pharmaceutical preparations.