Back-bar and retail should move together. Every Swedish massage deducts 20 ml of base oil + aroma; every classic facial consumes one ampoule + two pads; waxing deducts wax by gram and strips by count. Retail (SPF, hyaluronic serum) tracks by batch/expiry; tea station or pool bar smoothies can be tracked as simple recipes too. Purchase orders replenish low pars, and waste logging explains variance. Reports show theoretical vs actual so you can fix leakage before GP suffers.