I would make sense to come up with a calculation to figure BR...
(Weight of broadside + armor of ship)/some ratio= BR
yes, I guess that would mean that different builds of a ship would result in different BR values but at least it would be rational. otherwise, we're just making an opinion call.
Personally, I think all ships of a class should have BR within a few points of each other. So all ships that were 3rd rate ships would be nearly identical depending on broadside and armor.