I believe it's at least 3% of the damage and maybe be one of the 12 top DPSers.
Also take into account, if your farming the same boss repeatedly there is a short cool down till when you can loot the boss again, for most bosses this means you can only loot the same boss every 2nd time it spawns (if camping the one boss) no matter how much damage you do.
Also try dropping a high damage ultimate as soon as the boss spawns, might get you over that 3% threshold.
There's probably a % of damage needed indeed.