Just remember: when you are testing your skirt when you are sitting, don’t pull it down a lot before you sit because that’s just not honest. Just sit down as you always do. That makes sure it’s completely tzniusdik – yes, even if it’s 3.5 inches below the knee. It might be just fine. I once had a skirt that was 2 inches below the knee but it was still tzniusdik (until I grew of course ;)).
