What are the Thresholds for Killing and Looting endings?

[Godspeed]

You get a rating for killing zombies and you get over run if your rating was low

A similar ending pops up depending on how well you looted stuff

But they just grade you with letters and there is no way to tell how to reach each grade/letter.

Got a D for looting efficiency and a D for zombie killing but i have no idea how long im supposed to wait around on each map killing extra zombies for A.

[SEND 'EM TO THE RANCH]

Same here, I got an A for looting efficiency and a C for zombie killing. Maybe if there was a specific mission near the end of a run that had a reduced wave timer (or greatly increased spawn count per wave) compared to your normal wave time and all of the random npcs that you saved to help you out. That way you could farm up a lot of zombie kills quickly instead of waiting forever and more than doubling the time it takes to complete a run.