Id wait until January to shop, there shouldnt be much weight left, but explain to the shop staff that you need some size flexibility and that your final measurements are unknowable until the wedding date. You cant know what youll weigh in September, maybe goal weight, maybe somewhat above or below. For my own wedding i predicted my weight loss; the shop doubted me yet went with my number. It came out perfect, but nerve-wracking, given i never tried the dress on beforehand.