What factors are considered to be "general practice" in a tort action?

Klar, Tort Law (5th ed. 2012), supra, pp. 355-59 also discusses these factors, then (pp. 363-5) comments on industry practice: 5. GENERAL PRACTICE A very useful factor in determining whether a person’s conduct was reasonable or not is the general practice of those engaged in a similar activity. ... Evidence of general practice will be given more or less weight depending on the circumstances. There are no specific requirements to establish a generally approved practice. ... The burden of proving that a custom exists is on the person alleging, and “only in the rarest and most patently obvious cases will the Courts take judicial notice of a custom” [quoting Iacobucci, J. in Waldick v. Malcolm (1991), 1991 CanLII 71 (SCC), 8 C.C.L.T. (2d) 1 (S.C.C.), at p. 15]. Under the previous, but now discredited, theory that evidence of general and approved practice was conclusive proof that the defendant acted reasonably, it was of course important to have a fixed idea of what constituted general practice. Under the current, more realistic, approach that what others do is only a relevant factor in deciding what is reasonable, the weight of which varies depending on the circumstances, it is unnecessary to attempt to define the elements of a general practice with precision. ... It is almost universally conceded that evidence of general practice can never, as a matter of law, settle the negligence issue. Thus whether the evidence is that of the plaintiff, i.e., that the defendant failed to adhere to general practice, or of the defendant, i.e., that the defendant was adhering to general practice, the evidentiary effect is the same. This may raise an inference of negligence, or of reasonable care, or may in fact raise a prima facie case, but will not be conclusive proof.