We take a different approach:
We're at the point in our lives (age and financially) where we 'spend what we spend'.
We know intuitively what we can/cannot afford.
We don't buy stupid/crazy stuff.
We spend on what we need and want which has been fairly consistent year over year.
If we need a new roof, we get a new roof but that doesn't stop us from going 'over budget' and still go on that vacation.
Our approach is to tally our total after tax income. SS, dividends and IRA W/Ds taken as needed.
We assume that there is a "zero" balance at the end of the year, which is almost always close enough to true.
Simple, easy, and generally requires about 5-6 lines of addition.
Surprisingly (or not), the totals have been fairly consistent for many years. Spikes can be pointed at as "new car/major home repair/special trip to Italy etc." and treated as outliers.
Money in minus year end balance=spending.
Living well is the best revenge!
Retired @ 52 in 2005