I think short of having a wet suit company making you a custom fit vest, proper fit is mostly luck, finding the manufactured vest that fits your individual dog best. The dog in my avatar is wearing a vest that O'Neill wetsuit company made for our dog back around 1995. Unfortunately they won't do that anymore. At the time they would do a minimum order of five vest at $75.00 per vest. We ordered two, one for each of our dogs along with vest for three other friend's dogs. That vest is still usable today, but it doesn't fit either of my two current dogs.