I also recently moved up to China. I figured out the obvious way to tell: when you're finishing Scandinavian events in less than 1/3 of the allotted time, it's time to move up. This has the nice advantage of being completely adjustable to your style of play/available time, rather than just looking at other people's numbers. Though if you're curious I moved up at 9800/hr day and 40000 night capacity, and I'm feeling perfectly happy about it.
Hoarding food without purpose is pretty bad. If you can turn it into plot advancement/equipment you should be doing that, because those things will pay off with more energy in the long run. It's good to keep a small amount of food for emergencies, or for something like Builder which is harder to max out than same-region events. But sitting on tens of millions is just hampering your progress.
