Concerning the leg press, you have to remember its inclined at about a 30 degree angle, so you are only having to push against 50% of the weight. Also, you aren't having to lift your body up with you legs so that shaves another 100-200 lbs off.
So the equivalent of squatting 315, would be 2X315 plus another 200 lbs on the leg press. Which comes out to 830lbs.