I heard, though hardly official, that you need to be above 300 for a chance at 310 exotic engrams and above 310 before seeing 320 exotic drops. I just hit 310 (haven't tackled the raid on hard mode) so I'll keep my fingers crossed but it's a good thing I don't play exclusively for the rewards.
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If you're above 310, the dropped exotic is guaranteed at 310. There are no 320 exotics (yet). The longer you play, the more you play for rewards. I and many others try to minimize (Time + effort)/rewards...
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Then I definitely need one more light level because I picked up an exotic engram while running the daily mission that turned into a 290 4th Horseman which I promptly dismantled. LOL.