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.