Ive been working at just below knees to strengthen my weak point, but would a shorter range of motion be better so I can overload the deadlift with a greater weight than normal?
It depends on the lifter, the weakpoints, and where you are in the cycle. There's really not much point in handling more than 120-130% of what you can pull from the floor.
BTW, I rarely visit this forum anymore (this is the first time in about a month). I'm usually at the new website: www.ReactiveTrainingSystems.com