Largest line
Housing
Rent or a mortgage payment is usually the biggest single number in the plan and the slowest to change. A lease runs for a year; a sale takes months. That is why a budget built around an unaffordable housing line usually fails in the other categories first, as groceries and savings absorb the difference.
If housing takes more than roughly a third of take-home pay, the rest of the plan has to be tight by design rather than by accident. Naming that trade-off early is more useful than discovering it in month three.