I think that “always give some Fever” might be a good place to start. (Right now, it's directly proportional to Coherence, so shooting with zero Coherence gives zero Fever).
I see, that is a bit harder to intuit as a player because I conceptualize bullets as distinct things once they leave my ship instead of being treated as a group where one hit “validates the rest”.
Actually, they aren’t a group at all. Firing starts coherence drain and any bullet hitting any target stops it. (The state of this is visualized as the wavy part of the bar — each of the 3 quantities has a “temporary value/modifier” associated with it which is drawn that way, but each one has different rules.)