It's a set percentage per mob (supposed to be, at least, whether there's some way to influence it like there has been with loot drops I'm not sure of).
The amount of merits awarded per merit drop is based of the rank of the rikti, one for minions, two for lieutenants and three for bosses.
And of course, the mothership raid rikti drop merits for every person that gets rewards for them.
It's 10% for all rikti mobs (apart from monkeys), excluding MS raid defenders, which are 100%. Minions give 1, Lts 2, bosses 3.
There is also 1 for completing any Vanguard mission.
I really should do something about this signature.
What determines the drops and frequency? It seems to be a bit sporadic and I'm curious. Somebody in game mentioned that it reflects the damage you do but that doesn't seem t be logical, given that tanks would get a lot less than blasters etc
Thelonious Monk