What is the legal test for severance in the context of unconstitutional statutory provisions?

The circumstances in which it is appropriate to make such a severance in respect of unconstitutional statutory provisions are set out in Schachter v. Canada (1992), 1992 CanLII 74 (SCC), 93 D.L.R. (4th) 1 at 11-12 (S.C.C.): The courts have always struck down laws only to the extent of the inconsistency using of [sic] the doctrine of severance or "reading down". Severance is used by the courts so as to interfere with the laws adopted by the legislature as little as possible. Generally speaking, when only a part of a statute or provision violates the Constitution, it is common sense that only the offending portion should be declared to be of no force or effect, and the rest should be spared … To refuse to sever the offending part, and therefore declare inoperative parts of a legislative enactment which do not themselves violate the Constitution, is surely the more difficult course to justify … Where the offending portion of a statute can be defined in a limited manner it is consistent with legal principles to declare inoperative only that limited portion. In that way, as much of the legislative purpose as possible may be realized … Therefore, the doctrine of severance requires that a court define carefully the extent of the inconsistency between the statute in question and the requirements of the Constitution, and then declare inoperative (a) the inconsistent portion, and (b) such part of the remainder of which it cannot be safely assumed that the legislature would have enacted it without the inconsistent portion.