The category collapse
Category budgets (lodging $X, food $Y) require bookkeeping nobody does on vacation — and they fail precisely when the trip gets good: the unplanned day trip, the extra night, the meal that mattered. The daily number survives because it is checkable in one glance each morning: running total ÷ days so far vs the rate.
Building the daily number honestly
Fixed costs out first (flights, tours), 15% buffer held, remainder ÷ days. The result — checked against the style bands — is the number to hold. Region-mixing trips average naturally; the daily rate handles Prague-then-Zurich without a second spreadsheet.
The on-the-road system
One line per day in a notes app: what was spent. Every 3–4 days, compare the running average to the rate. Early corrections cost nothing (a picnic dinner rebalances a week); late corrections cost experiences (skipping the last-city dinner to fix week-one drift). The daily budget is a rudder, not a cage.