TLC, yes wet has to be changed to dry matter. 
There is also a small amount of moisture in dry food and you can use the same equation to calculate it out but since there isn't much in it, it usually isn't necessary.   
Zanders' Mom, the drugs used for liver compromised dogs are often just supplements and don't really have many side effects.  Going with very low protein when it isn't indicated could be dangerous though.   
Hannah3183, is the vet against a Protein C test?     
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