In this great article, Moore suggests a practical approach to answer it:
First, simply track spending for a while to figure out what the monthly spend number is.
Next, take a hard look at the expenditures to figure out what would it really take minimalistically just to ‘survive’ in the current lifestyle.
Third, calculate the difference between the two – what you’re spending in practice, and what it would take at a minimum – and consider which dollars really, purposefully, should be put back into the spending budget to add to the (admittedly very) minimalistic lifestyle.
Finally, figure out how to adapt spending in practice to fit the (more ideal) spending pattern that’s now delineated.