Theoretical concentration assumes 100% recovery: 5mg in 2mL = 2.5mg/mL. Practical concentration accounts for reconstitution loss (2%), transfer loss (3%), and dead space (5%), yielding 90% recovery: 4.5mg in 1.8mL = 2.5mg/mL actual. Dose calculations should use theoretical concentration as loss factors are already accounted for in unusable volume.
USP loss accounting methods explain pharmaceutical calculation conventions.