Is some of the $13,000 employer match or pre-tax contributions deducted off the top of your income? Because those should be added to your net income number. Technically, any re-invested investment earnings should also be added to your denominator as well.
For me, the simplified method I mentioned works well because the bulk of my savings are coming out of my net income. If that's not the case for you, it may not be the best way for you to calculate your savings rate.
Like I said, the method will probably change once my investment earnings start making up a nigger portion of my savings. Until then, this method tracks what I'm interested in measuring.