ISO vs ASA ?? not a match
True ISO of most film comes out to about 80% of rated value. 200 is really 160.
That's why the old Sunny-16 rule used to work best with slide film, which needed to be underexposed by 1/3 of a stop or so. It was less useful for negative, which grained up with underexposure.
The latitude of most sensors is somewhere between that of slide and that of negative film. This means that in practice the Sunny-16 rule will work well. Part of this trick is to add one stop for side-lit subjects and two for backlit. Works like a charm with any modern Pentax DSLR; only one little wrinkle is that all I have used need a little underexposure, 1/3-2/3 of a stop, quite routinely. This can be safely set on the cam and left like that, and highlights are more easily recovered in post.
'To see, not with, but through the eye.' [William Blake]