Is 68 your wort temp or the temperature of the space it's in. I ask because fermentation creates heat and may be 5 or more degrees warmer than the room/chamber. If that's room temp, your fermentation temp may actually be 72-75, which is the high end of 1335's range.
I haven't used 1335 specifically, but I would guess that 65-68 is an ideal fermentation temp.