Exactly how is FATE Rating done?

Can someone explain all of the dynamics that are calculated and judged for FATE Ratings?

Sometimes I'll join a FATE with 90% of it completed, Whack a few mobs and get Gold.
Other times I'll be all but soloing the Fate and get Silver.


Me and an FC member were partied together doing the Corpse Brigade chain near Lil Ala. I had gotten there at the tail end of Part one and managed to Gold it after downing three enemies. Part 2 was pretty much the same. I killed a few adds and started fighting the fate boss. We got them down to 5 % and they ran away with 5 minutes left on the clock. I go further in and Part 3 starts. I engage the Fate boss with my FC healing me and I'm pretty much unloading everything I have on it, I drained my TP completely as a Lancer and had to rest it back up and take on a few adds. Came up with a silver Success even though there were only three or four of us in the area with two of us dedicated to fighting the boss down.

What is up with this?