The formula
Daily limit = (money left for living) ÷ (days until payday)
"Money left for living" is your income minus rent, bills, loan payments and what you're saving this month, minus anything you've already spent since payday.
Why it works
A daily limit adjusts itself. Spend less today and every remaining day gets a little more. Have a big day and the rest of the days shrink a little, instead of the whole month being "ruined".
Keeping it up to date
The hard part is subtracting every purchase as it happens. That is exactly what an automatic expense tracker does. Expensly logs each card purchase from your bank's alert or Apple Pay and updates today's number straight away, shown as a battery: green, orange or red.
Before a bigger purchase
Use a "Can I buy it?" check: see what the purchase does to today's number, to that category and to your savings goal before you pay.
