Well, I am not a scientist... so no scientific answers.
I basically tried 3 or so "basic" formulas and was disappointed at them petering out after a week or two. So I did a munch of research, internet, some old magazines, read some books at my
LFS, plus I looked a little bit at stuff for brewing.
The formula I use was basically me combining elements of several other folks formulas, that they claimed were long lasting. One person talked of adding a little Molasses, not for the sugar, but for some other trace element in there (sorry I'm at work and my notes are at home). Another person suggested some protein powder, this made sense to me, as it replaces say Grain in brewing. And the coco stuff was on sale, so I went with it. The rest (sugar/bakingsoda) are pretty common in most formula's. But several sources I read talked about how too high a concentration of sugar is also inhibitive to yeast. So the key there seems to be get as much water as you can safely put in your bottle. One last thing that I read several places was the importance of gently re-hydrating your yeast, before it goes into the full solution. Also, the need to let it begin its process exposed to air for the first day or so.
I don't wash the bottles between re-fills, and even try to leave a very small amount of the old formula in there to help get things started. (I only saw this advice from one source, but it also seems to "work")
So no real science per se, just a lot of research and using my own gut feeling to help me put it together. lol... actually that describes most of how I have approached this whole hobby. research a lot, ask a few questions, and then go with what feels right to me.