Four times Isaac's damage per fly
What changes
Rotten Baby generates a fly while you fire in a room with enemies, with one of its flies active at a time. Hive Mind strengthens that fly rather than increasing the familiar's production limit.
How to approach the setup
Keep firing from a safe position so Rotten Baby can send a fly into the fight. Value Isaac's damage upgrades alongside Hive Mind because the generated flies scale with that stat.
A practical suggestion for this pair. Check the rest of your inventory and the room before relying on it.
The caution to remember
BFFS! only makes Rotten Baby look larger; it does not boost these flies' damage. Do not expect the two upgrades to stack into another blue-fly multiplier.
What this teaches about a run
The familiar on the screen and the creature doing the damage can use different upgrade rules. Follow the damage source rather than its parent's size.