How much share of liability should a plaintiff bear when they are not wearing a seatbelt?

Lord Denning explained his reasoning in this way at pp. 527-8: 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 also was 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? It is proper to enquire whether the driver was grossly negligent or only slightly negligent? or whether the failure to wear a seat belt was entirely inexcusable or almost forgiveable? If such an enquiry could easily be undertaken, it might be as well to do it. In Davies v. Swan Motor Co. [[1949] 1 All E.R. at 632] we said that consideration should be given not only to the causative potency of a particular factor, but also its blameworthiness. But we live in a practical world. In most of these cases the liability of the driver is admitted; the failure to wear a seat belt is admitted; the only question is: what damages should be payable? This question should not be prolonged by an expensive enquiry into the degree of blameworthiness on either side, which would be hotly disputed. Suffice it to assess a share of responsibility which will be just and equitable in the great majority of cases. Sometimes the evidence will show that the failure made no difference. The damage would have been the same, even if a seat belt had been worn. In such cases the damages should not be reduced at all. At all other times the evidence will show that the failure made all the difference. The damage would have been prevented all together 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 the seat belt had been worn, but there would still have been some injury to the head. In such case I would suggest that the damages attributable to the failure to wear a seat belt should be reduced by 15 per cent.