It must be much higher than the 4% a typical American in your position has left. I'd be curious what the best case real world numbers are for people in your situation who are doing their best to optimize.

Which was on response to:

Personal anecdote: My own finances as well as those of six of my friends fall well outside those bands. (Emphasis added.)

You still seem to be missing the point. Statistical averages, even in percentage form, cannot be blindly projected across the entire income scale. Someone making $1M a year does not spend the same percentage on food as someone making $40k.

And what does it matter if he takes money out of his housing budget and directs it towards an expensive car? The point of disposable income is that you get to spend it on things you want. That includes nicer housing, restaurant meals, and fancy cars, the value of which is ignored in your calculations.