If you're continually missing PR attempts off the floor, try using block pulls off 3-4 inch blocks. This will allow you to challenge your hip and back position at an angle that mimics where the bar will be right after you get it off the floor.
Experienced lifters should do these with a quick set-up and a rapid descent.
So you can squat 405 pounds and bench 315. Is that good? Does that make you strong? How do you compare to others? Find out if your lifts are decent, good, or great through Tim Henriques' handy little comparison chart.