I think you'll see a lot of variance and "that's not how I do it" here...
I have always used:
$percent=($income - $expense) / $income * 100;
In other words: it's the amount of your income you keep expressed as a percentage. In my case, I include all income: salary, bonuses, interest, dividends, capital gains, etc. The only thing I explicitly ignore in my calculations is "found" money. For example, gifts, etc -- I exclude those as I have no expectations of them being any kind of reoccurring event.
I actually calculate this for 30days, 90days, 365days and 5years. (I do this for most of my calculations). I try to spot good/bad trends and try to use the longer periods to see that they pan out.