At what point do you start farming just level 10 NPCs?

My Upkeep numbers are currently at shy of 15 mil for almost all of my cities on most of my servers...I farm a fair amount of NPC10s and even more NPC5s in trying to keep a decent Food buffer in case I have to miss or cut a day short.
I am looking to break into that next level...but am unsure on exactly what needs to be done.

-cfcrew