The simple solution would be to advertise full spool max drag capacity and (near) empty max drag capacity, to be completely transparent.
Agree that it could be, and probably is a selling point that may sway someone towards a particular brand, but you would have to assume that the other large manufacturer is measuring and stating their max drag capacity in a similar manner.
I read somewhere a while ago the difference in max drag between full spool and near empty spool, and it was over double the kg's, someone may have an equation
Scott.