Well, I will simply report on what I have seen in Kodak's literature. For years, Kodak recommended a scene lighting ratio of 3:1 for transparency work. I beleive that assumes the use of reflectors, lights, strobes, etc., for carefully managed light. I do not know where one would normally find light like that out-of-doors without some serious lighting setup. IIRC, that translates into ~1.6 stops. It probably represents a figure that would allow for just enough shadow to make things interesting without straining transparency film.