You can run the numbers relatively exactly. 35% of income is spend on housing, which "disappears" (we can assume this will support some banks and realtors or whatnot), some is spent on this and that, and the remaining spent on "household, groceries, entertainment, etc" would be what can support retail shops. Then you just need to know how much it costs to run a shop, and you know how many houses you need to support it.