How it works
Expected spending is every person's gift cost plus the other costs. What's left is your total budget minus that. The amount you need by the day is the larger of the two (if the plan runs over budget, you still have to pay for it), minus what you've already saved; divide it by the months left to get the monthly amount. Months left counts from next month to the holiday month, so each month's saving comes out of a paycheck you haven't spent yet: from late September to 25 December that's October, November and December. Someone is over budget when their gifts cost more than the budget you set for them.