No need for meters. How about this:
Subs did not have a store meter on board. But they did know they had enough food for x number of days. Food consumption was pretty well a constant (crews may have eaten more calories in the northern regions, but this is negligable for game play).
So, when your leaving port, decide how may days of food you've taken on board (probably 30-45 days worth, you decide), note your mission start date, establish what your "return by" date is, and make sure you return to port, or meet a resupply vessel no later than that date.
GWX gives you tools to calculate transit time, and this can be used to calculate and update your turn around date (farther from home means earlier turn around due to longer transit time).If you find yourself turning to head home with a load of torpedoes still on board, and few if any kills, because your "return by" date is approaching, you will have experienced what many U-Boat skippers did.
The fixed "Return by" date is a great idea for added game play realism, and no food meter is required to add this to game play.