When to Order Wedding Dress

I'm in the middle of losing weight and really don't want to order my wedding dress until I'm 10 lbs from my goal weight. What is the absolute latest that I should order my dress? I've heard 6 months but that seems way to close to the wedding.

Also, what's an ok amount of weight to lose and still have the dress fit properly after alterations for additional weight loss? 5lbs? 10lbs? 15lbs?