Thanks for your quick reply.
It's the second time within a week a mod author said that. xD
Maybe don't handle that case at all and leave it to the user to decide how much ammo they want?
I would assume if someone would fill the turrets with inserters, they'd set the value lower, but also that in that case the mod itself would provide less value, because when you automate it, there are less chances for mistakes than when you fill them manually. (Which I still do, because I'm not yet ready for the automation.)
That said, to keep the mod as closely to what it is right now and not confuse others, a solution might be adding a setting (bool), that is set to true by default, that people who fill manually can turn off, and handle that edge case only when the setting is not active.
That's all I can think of for now.