bridggar:

I'm really curious as to how they'd calculate the cost of use of the vehicle as I suspect I am going to have to go down this route with Porsche for my problem child 991. 

I don't have the professional background or relevant personal experience to help with that question. Considering what you have told us here about your issue you'd probably be well-advised to contact a lawyer specialising in consumer protection matters. 
Not trying to introduced semantics into this, but by my understanding its not an issue of "cost of use" but of a "provision for benefit" accrued to you during your ownership, and in your position I'd be inclined to argue a case for the "benefit" having been seriously impaired due to the dealer(s) inability to fix the problem despite your complaints over an extended period. 


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fritz