It's basically the same as Luck formula.
Hero item drops + Hirling Item drops /2 . Then add by +100% bonus Item drops over cap.
For example if main hero has 250% item drops and the hirling also has 250% item drops , it's 500% item drops.
Then 300% item drops /2= 250% item drops. That's if you used Epiphany . With item drops bonus of a 100%, that turns into 350% item drops which is possible.
You would need Epiphany on both characters for this effect. If however you didn't use Epiphany, cap is 200% item drops but with 100% hirling item drops bonus, it's 300%.
How to get? 200% item drops per character /2= 200% . That's 400%/2= 200%. 200%+100% bonus = 300%.