Very helpful!!! In particular there's 3 different things that I didn't know until reading this article
1) CS's prefer to be near R's of the same wealth class
2) CO's prefer to be near CO's of the same wealth class
3) Demand isn't just for "overall residential," it's for a certain wealth class (i.e. R$$)
In particular, I think #3 will help me out a lot. I've been so good at blanketing my cities with perfect education/health-care that maybe there's too few R$ for some new commercial businesses .... or something like that. Anyway, thanks!