The idea of a savings rate is to create a ratio between the amount you're saving towards retirement and your total income because this implicitly tells you how much you're spending, so having the dependent care and 529 in the numerator don't feel right to me. You're not going to spend them in retirement. On the other hand, they do represent a reduction of spending (in retirement you don't need to replace the income that's going into the 529 assuming you'll save enough by the time you retire). The dependent care account isn't "real" savings, just deferred spending -- will those expenses continue in retirement?
Assuming you'll be done saving for college once you retire, and the expenses paid for out the dependent care account will go away (you're not working, so you stop using daycare), then I would be comfortable with something more like:
(401k contributions + IRA + Mortgage Principal Payments + Other Savings) / (Gross Pay - Dependent Care Savings Account - 529 Plan)
If the dependent care spending will not go away, then just leave it out of the calculation entirely:
(401k contributions + IRA + Mortgage Principal Payments + Other Savings) / (Gross Pay - 529 Plan)
At the end of the day, it doesn't really matter how you calculate it, it's just a way to measure progress against yourself so you know how you're doing over time. If you're trying to look at the "this many years to retirement" type posts like MMM's Shockingly Simple Math, then I think those tend to use take home pay instead of gross pay and then add back in things like the 401k contribution (that are both income not reflected in take home pay, and savings) to both the numerator and denominator.
Since I track expenses very closely and total savings for retirement is easy to figure out (beginning balance - ending balance), I like to take it from the opposite side and calculate the multiples of expenses I've saved in a year. This gets back to the 25x (or 30x, or whatever multiple you're comfortable with) that you need to save to retire. Even with this you need to remember that there are likely to be expenses in retirement that aren't reflected in your current spending (or vice versa) like health insurance.
