Destiny: The Taken King's toughest mission, the King's Fall raid, requires a light level of 290, reached by unlocking high grade legendary and exotic engrams.
However, the best time to decrypt engrams isn't when you hit 290+. Redditors have calculated that the most 290+ decryptions occur when you reach light level 281-285, at 19%. Conversely, the fewest (7%) occur when you are between light level 291-295.
You still have a pretty good shot at 286-290 (17%), while at level 296-300 you have a 13.25% chance.
In other words, if you want to unlock the most 290+ engrams, you need to use lower level equipment of you want to maximise your chances of picking up a high level engram.
For more on Destiny: The Taken King, check out our numerous guides and the ten best moments in the game.