You can model a lot of stuff, but at some point it starts becoming impractical. Few people are even gonna have a /g/ story setting of that scope (in fact, dealing with millions and billions of people wouldn't be a guro story as much as a bunch of statistics; if anything this sort of math would be far more useful if you're dealing with something like a human 'farm'/breeding setting with a few 100 to 1000 women), much less one with that much thought put into every single variable. On the fly I can give you a far more basic formula:

Population of the next year = Population of the current year + (Population of the current year * birth rate in %) - (Population of the current year * death rate in %)

* Just leave out men for now (since you don't seem to consider them guro 'targets'; I don't do male gore either), so we only have to deal with a single population .
* Posit a pregnancy takes 12 months because it's easier to math. You seriously don't need to consider the fathers on such a scale.
* Notice that (outside of small breeding populaces) a birth rate of 25%/year is abnormally high, the worldwide average is below 20 births in every 1000 women. Similarly the death rate is like 8.3 in 1000. (I just googled for those numbers, no idea how recent/accurate they are).

You can do two things with that: Either you get the 'surplus' each year, that is, the absolute number of women you can still kill while maintaining the population. That's just gonna be Current Population * (birth rate-death rate %) each year. For example: 1.000.000.000 * (0.05-0.01) = 40.000.000 Kill less, and your population will grow, kill more and it will decline in the long run.

Or two, you can change the rate of deaths to include all deaths. In that case, as long as it's below the birth rate, you'll be fine, if it's higher, the population will decline, (and if it's exactly equal it will stagnate), no need to even figure out the total if you just want a prognosis.

But for anything much more complicated you'd need a spreadsheet with a bunch of categories for age groups and birth/death rates for every single one. It wouldn't be so much hard as a bunch of work. I might just try, if you'll work with me a little, discuss stuff like the various age groups and birth/death rates, etc. (which we can probably just do in this thread; it'd be grand if you could create and link a GDocs spreadsheet)