Wat SRS says is spot on.
For me, I have a Metal ACE suit. I have box squatted in the mid 5's with the suit on, straps down and a belt. Throw the straps on LOOSE (as in hand high spacing) and I've hit 655 in the past. Straps semi snug on the shoulders and I had to go to 695. NO fucking way I coujld go tight with them. I've been using just briefs for some time now...I should see what I can get in the ACE now...that's another thread tho.
For what it's worth when I used my single ply suit, my raw half squat off a box was the exact same as my suited full free squat. I'd use this as an "indicator" of where I was at.