WB,
When doing Power Squats you want to make sure that you are clearly breaking the plane. I tend to drop to parallel and then sort of open my hips up a bit. This allows the ass to drom another 3-4 inches and gets that real deep squat.
I would reccomend starting with a light weight until you are comfortable with the movement though. I have found that the first few times you do this correctly the drop is a little disorienting and can cause some forward or backward sway. Bad news with heavy weight.
If I can offer any more help feel free to pm me.
Gig