Very simple test code (literally less than 20 lines) in Testing now. It shows you your current provision level, will only supply inside your own realm, and the calculations are very trivial (+10 % per turn to resupply, -20% per turn whenever you are in a region that has no surplus. That means that you use up provisions when there is no starvation, but the population needs to be fed from the warehouse - i.e. most cities. But your troops will not starve as long as the region is not starving.
