How is a plaintiff's capacity to earn income calculated when they have lost their capacity to work?

The court in Pallos v. I.C.B.C. proceeded to comment at p. 271: The cases to which we were referred suggest various means of assigning a dollar value of the loss of capacity to earn income. One method is to postulate a minimum annual income loss for the plaintiff’s remaining years of work, to multiple the annual projected loss times the number of years remaining, and to calculate a present value of this sum. Another is to award the plaintiff’s entire annual income for one or more years. Another is to award the present value of some nominal percentage loss per annum applied against the plaintiff’s expected annual income. In the end, all of these methods seem equally arbitrary. It has, however, often been said that the difficulty of making a fair assessment of damages cannot relieve the court of its duty to do so.