This is a timely thread for me.. I just started using the squat rack at my gym about a month ago and its great to be able to go so heavy now. (been doing the Leg Press and Smith Machines on my leg days and this has been a fun new challenge to conquer.. its very different than either of those machines...
I can do 135# right now 3 sets of 8. However that is only when I'm feeling pretty strong and energized that day. I've noticed that at that heavy of a weight ( for me) at the last set I will cheat the move and not go down as low as I should. I usually back off to about 115# and my form is much better.
Start light and really focus on your form. When I first started them, I was surprised at how much of your core comes into use trying to maintain your body upright with such a heavy weight. Also, the first couple of times you try them, maneuvering around with the bar feels pretty awkward, so really make sure you take your time and focus on your leg and hand placement before putting the bar on your shoulders.
If you have a trainer or spotter, have them check you for proper alignment the first few times.. you'll be playing around w/ a lot more weight, you'll want to make sure you are solid with your form.
Take care, Lynn M.