I lost 5kg prior to surgery and have lost another 3.2kg in the 6 days post surgery. I am very happy with my weight loss so far as any loss is still a loss.
Don't stress too much over what you will lose before surgery. The most important is that you do the optifast as your doc suggested as it not only is there for weight loss, but to shrink the liver for surgery and to prepare you for the weeks after surgery.
Remember everyone is different and will lose differently. And again, any loss is a loss and it will never come back again.
Keep up the good work Aus_girl..