When I powerlift, I do two light warm up sets (135) for about six reps each.
Then I get into my bench shirt and do one light set in the bench shirt, which for the tension in the shirt, is about 225 or so (anything less and there isn't enough downforce to touch your chest)
This warm-up routine is pretty standard. So, in your case, without a shirt, I would do two warm up sets @ 50% for six reps each, then do a single rep at 85% or so, so you can feel approximately what to expect. Then go for it.