Build the number with three lists
Start with the household’s net monthly income — what actually lands in the checking account after taxes, retirement contributions, and benefit withholdings. Not gross pay, not last year’s W-2 figure, not the pre-tax salary line on the offer letter.
On the other side, list two kinds of outflow. First, the recurring fixed commitments — rent or mortgage, loan payments, insurance premiums, contracted subscriptions. Second, the regular variable categories — groceries, fuel, utilities, dining out, household supplies. The distinction matters because the variable side is the side that has to be sized against an upper limit, while the fixed side is sized against the contract.
The third list is the irregular outflows: annual insurance renewals, estimated tax payments, holiday travel, medical out-of-pocket, predictable maintenance. These get spread into monthly sinking-fund contributions (covered in lesson 03), so they do not show up as surprise drains on the budget when they finally land.