How much is a plaintiff's contributory negligence when they are not wearing a seatbelt?

Lord Denning in Froom v. Butcher, supra, dealt with the question of apportioning an injured plaintiff’s contributory negligence where it was established that the injuries were in part attributable to a failure to wear a seat belt. He concluded: Whenever there is an accident, the negligent driver must bear by far the greater share of responsibility. It was his negligence which caused the accident. It was also a prime cause of the whole of the damage. But insofar as the damage might have been avoided or lessened by wearing a seat belt, the injured person must bear some share. But how much should this be? … Sometimes the evidence will show that the failure made no difference. … At other times the evidence will show that the failure made all the difference. The damage would have been prevented altogether if a seat belt had been worn. In such cases I would suggest that the damages should be reduced by 25 per cent. But often enough the evidence will only show that the failure made a considerable difference. Some injuries to the head, for instance, would have been a good deal less severe if a seat belt had been worn, but there would still have been some injury to the head. In such a case I would suggest that the damages attributable to the failure to wear a seat belt should be reduced by 15 per cent. (pp.527 and 528)