If you assume a 4% SWR, every $1 you spend monthly means you need to save $300 to cover that expense forever. So, if you save $13 per month, that's $3,900 less you need to save. Of course, that will also compound as it helps you save more that will grow on it's own earlier to further reduce it - but that really complicates the problem. When I'm doing it in my head - I just take the monthly expense * 300 and divide by the amount I typically save in a month.
This is basically what I do for recurring expenses: frame it in terms of
how much bigger does my stache need to be to support this expense indefinitely? Quick and dirty approximations, based on SWR, are as follows:
4% SWR: yearly expense: multiply by 25, monthly expense: multiply by 300
3% SWR: yearly expense: multiply by 33.3, monthly expense: multiply by 400
2% SWR: yearly expense: multiply by 50, monthly expense: multiply by 600
After doing the multiplication, it should be easier to determine how much longer you have to save to meet those goals. Likewise, going the other way, for every recurring expense you eliminate, you can derive how much you cut off your stache-building time.
As JohnGalt said, it's hard to use such rules for one-off purchases. But if you track your spending precisely, you could, for example, tag all purchases like this, then add them up every month, and frame them as a recurring expense. If you have the discipline to order pizza for dinner
only once a year, then it will have virtually no impact on your time to FI.
But if you're like those of us who are still working on such discipline, you don't order pizza just once a year. :) Add up all the times you got pizza or ran through a drive through or whatever. Say you do it only two or three times a month on average. It could still easily come up to $30/month. If you're targeting a 3% SWR, that's $30x400 = $12k of additional stache you need to support it indefinitely.
