When actual weight is the wrong number
Weight-based dosing has a catch: fat isn't very vascular and doesn't take up most anesthetic drugs the way lean tissue does. For a lot of drugs, dosing a large patient on total body weight overshoots — sometimes badly. That's where ideal body weight comes in: an estimate of lean mass from height and sex (the Devine formula), used in place of the scale number so a tall or heavy patient gets a dose that matches their physiology instead of their bulk.
Infusions add one more step
A bolus stops at the dose. An infusion doesn't — you still have to turn that dose into a pump setting. It's the step that eats time and invites arithmetic errors mid-case: dose (per kilogram, per minute or hour) times weight, divided by the concentration in the bag, to land on mL/hr. The logic is simple; doing it under pressure while everything else is happening is not.
Where Master Anesthesia comes in
Master Anesthesia does this reasoning for you. the In-Post Dose Calculator lives inside every drug page — set the weight, pick adult or peds, and it applies per-kg dosing, swaps in ideal body weight for the drugs that need it, and works out the infusion rate in mL/hr from the concentration. The clinical logic above, done in a tap.