Hannah, I go by my SW weight, and I make sure I wear the exact same clothes each week to group so that there is never any fluctuation in the weight of what I am wearing. That way, the weight loss I see is completely coming from me, and not my clothes!
Also, I officially dropped into this zone last week, but didn't get around to posting. This week i am firmly here, having dropped to 17st 8lbs at weigh in today.