IMO nothing should be done 12 hours out. Everything you put into your dog will be dead weight. All you can do is let your dog drink water until a couple of hours before. The reason we empty our dogs before a hunt is not only to loose weight. Dogs are programmed to hunt empty. Thats what I do. If you are under weight you did misjudge your dogs weight. If you believe it will be a big disadvantage pay ff. If not go ahead. Maybe now you know his true best weight.