I track that information in a half dozen major categories (ie: housing, vehicles, food, tec) and a few sub-categories for each of the main ones.  Most banks offer the service as part of your accounts.  So it is not difficult, but yes, it requires some time...maybe an hour or two to set it up and monitor it.

One of the points the article fails to mention is which major iterms to subtract from income.  Should one subtract annual insurance payments?  What about utilities?  Muni taxes?  Given all these questions, and given that you need to consider them in this new calculation then why not just set up a spreadsheet with all expected period related expenses?  Good heavens!  That's what I do already!