People usually underestimate water because they think mostly about drinking. In reality, cooking, dishwashing, quick cleanup and handwashing often consume a surprising share of the total.
A practical water plan separates drinking from utility use. Once you do that, the reserve margin becomes easier to reason about and you can see whether your storage and refill assumptions still work.
For hot weather, family trips and remote routes, adding buffer matters. Running low on water changes comfort, hygiene and safety faster than many other camp resources.