I was thinking the same thing... o.o
I've heard a theory (I believe it was Thevedis'):
<10k = 1 drop
10k-?? = 2 drops
Dunno about after that. I can confirm these numbers though as within those ranges I've seen exactly that amount of Dark Matter drop every time. I'm thinking it's like Primals, either Dark Matter or Gear.
Edit: As for the actual % chance drop rate itself, I have no idea.
People regularly broke 40,000 points and got less than those at 10,000 points. This is central to the issue, lol.
Drop rate of something other than trash from captain: ~ 25% [top 20 exclusive]
Drop rate of something other than trash from chest: ~ 0%
again, this is the problem we are referring to, lol.
I think there may be some correlation. I was wondering if you have a top 20 contributor (3000 points), plus you beat the level 2 boss (3000 points), plus your frontline was unbreached (1500 points), and maybe a few more high point value occurrences, if the rest of the party would have better chances.
I won't risk the loss of gil, personally. Maybe after the other two open up, so there is a better chance to make it into the top 20, but not before then.
Also, multiple top 20s, as far as I can see from the data I have, don't count for more points than a single top 20 in your group would.