Dude, you got some potential troubles there.
First of all, you looked like you almost missed the box on one of those reps. That would be disastrous to your spinal health, to say the least.
Second, and almost as important, you have to sit BACK and not down on the box. When viewed from the side, a good squatter's knee's will never move forward out over his toes. Knees must stay in the same spot as when you are locked out. Think about sitting back to the point that your knees are actually farther back when sitting on the box, than they were while you were standing. It can be done, but only with a box squat.
I would also suggest to anyone doing box squats, that they invest in either a squat suit, and leave the straps down, or preferably, get some good quality breifs. These will really save your hips, which will take a terrible beating if you are squatting wide and deep.
But, you are box squatting, which is good. Only way in my opinion to know you are going deep enough every rep is to do it to a box.
One last thing, make sure you always sit completely down on the box, which it looked like you were doing. Don't just squat down till you just touch the box then spring up. Sit down completely, relax hip flexors, while keeping every thing else tight, then spring back up as fast as you can.
Keep at it, and work on those things, before you know it, you will have 4 or 5 plates (or more) per side on that bar.
B.
Please Scroll Down to See Forums Below 
. I don't know if you can get anything out of it - it only shows my legs.
