What is the test for a reasonable defense of habitation?

California law declares to be justifiable, homicide committed in reasonable defense of habitation (Pen.Code, 197; People v. Gleghorn (1987) 193 Cal.App.3d 196, 200, 238 Cal.Rptr. 82), but it extends the presumption of "a reasonable fear of imminent peril of death or great bodily injury" only to residents (Pen.Code, 198.5). This makes sense. It is a judgment rooted in our veneration of home and our recognition that the resident is in the best position to assess an ostensible threat to the home, consider any extenuating circumstances, and determine what response is necessary.