After months I have finally figured this out (I'm slow, but I'm good
Say you want 14 days worth of ferts. You add together the doses for 2 weeks. That's how much dry stuff you need. You take your pump and figure out how much water it dispenses in say, 1 minute (or 2 minutes, or 3 minutes, however much) when it's set up exactly as it will be with the fertilizer solution. Say that's a half a pint (one cup). Mix 14 cups of water and the 14 days worth of ferts in your container, set your pump for that time where it will dispense one cup a day and you've got it. Of course, that is just for 14 days. You'd probably either want to mix up a couple of extra days to take care of that problem you're talking about, or change it out when it's getting low (say at 12 days instead of the 14 day mark.) I know there's some pumps that say
they'll pump in something like a half inch of liquid, although I don't know if I'd really want to test that or not!
Just happened to think, alternatively what Wasserpest has done I think is to fill up his container with water, run the pump for a minute, make a mark, run it for another minute, make a mark, etc., etc. If the container is tapered the marks will be at different heights. It'll give you the same measurements sort of. After you have all the marks you'd know to fill it up to the 14th mark and add your two weeks worth of ferts. Same thing.