"[5.5.1] Factual difficulties. Where someone is self-employed, or conducts a business through a company or partnership, ascertainment of the loss sustained as a result of injury to that person often presents considerable factual difficulty. Records of earnings are not always maintained in accordance with best accounting practices; annual income tax returns in previous years, if available, frequently show variations from year to year; rates of expansion are not easily predicted; losses could be due to a general economic downturn. The problem is aggravated where the plaintiff has not yet commenced the business, but intended to do so prior to the injury. Sometimes, notwithstanding the plaintiff's absence through injury, profits of an established business are not less than in previous years and it is impossible to say what difference, if any, the plaintiff's presence would have made. The mere showing of a loss does not necessarily mean that it is recoverable by the plaintiff - or even by anyone else. The court has to do the best it can in such circumstances to measure the loss that is due to the injury. The amount a self-employed person drew by way of salary from a business may not be a true reflection of earning capacity. The requirement to mitigate the loss would ordinarily mean that the damages under this head cannot exceed the cost of employing someone to do the work that the injured plaintiff is unable to do. In some instances, resort may be had to what the plaintiff could have earned as an employee elsewhere, particularly in relation to the future because the plaintiff's earning capacity would not necessarily be tied to a particular business. What the plaintiff could earn as an employee is no more than a guide, since, on the one hand, persons who devote themselves to unprofitable businesses apparently cannot recover more damages for loss of earning capacity than they would have derived from their businesses if they had not been injured, and, on the other, a business may produce extra profits for the proprietor." (emphasis added)