How can you tell exactly how many materials you need to send to keep your armies running? I know that if, in the convoy screen, your convoy data is red, that means the convoy won't be effective (though what this is supposed to abstract or represent, I have no idea). But sometimes, even when all the convoys are operating properly (all white text, no red), I see that my armored and mech and air units are not getting oil. If I up the number of conovoy transports significanly, then their little oil icon turns blue, and all is well. But I don't want to create bigger convoys than I have to, or send more oil than I need to. How can I tell the minimum that is needed?