Typically rack deads are done from either just under or just above the kneecap. You should be able to pull the bar to lockout without a significant change in knee bend, maybe 20 degrees or so. Any more and you're putting the legs into it and defeating the purpose.
Although for a change you could just set the safety pins at 3-4" under lockout and throw a shitload of weight on the bar, just to get yourself used to straining with heavier weights. I know when I've done that I can lock out a good extra hundred pounds or more beyond my full-range deadlift.
