It's not necessary to murder everything in the FATE to get an ATMA, and you only need a certain percentage to get gold if you're farming seals as well. Just like the turn in FATES where you only need to turn in 8 items for gold. When I was doing the low level FATES and new players showed up, I'd just get my percentage of the completion bar and move on to the next one. Since you don't even need gold to get an ATMA drop it's in your best interest to run around the zone and get hits on multiple FATES to increase your chances of a drop.