Start from what really fits
Record your available net income, not your gross salary or an unpaid invoice. If income varies, look at multiple months and build a conservative base. An occasional refund can help you, but it's not a good basis for taking on a new recurring expense.
Divide the outputs by function
A useful distinction is between essential, discretionary and periodic expenses. It does not coincide perfectly with the division between fixed and variable: food spending is variable, but remains essential.
- Essentials: housing, utilities, food, necessary transport and installments due.
- Discretionary: outings, postponable purchases and non-essential subscriptions.
- Periodicals: insurance, maintenance and other deadlines that don't arrive every month.
For an expected annual expense you can set aside a monthly amount. If the deadline is near and you haven't put anything aside yet, divide the amount by the months actually left instead.
An example, not a rule
With €1,800 of income and €1,400 of recorded expenses, the apparent margin is €400. If you have to prepare €600 insurance in twelve months' time, a quota of €50 per month brings the margin not yet assigned to €350.
This amount is not necessarily all spendable: also check for unexpected events, other deadlines and objectives.
Close the month and adjust the plan
Compare forecast and actual spending. If an entry frequently exceeds the limit, the problem may be an estimate that is too low, not a lack of discipline. Update the amounts and choose a concrete change, such as reviewing a subscription or scheduling a purchase.
If expenses steadily exceed income, first distinguish obligations from deferrable expenses. If you have debt difficulties, seek qualified assistance: a spreadsheet alone does not solve a situation of over-indebtedness.