Dear David,
1/ISO @ f/16 e.g. ISO 125, use 1/125 @ f/16, or ISO 400 use 1/400 @ f/16.
It worked a lot better before 1959 when film speeds were redefined and effectively doubled overnight. Hence my preference (widely shared) for sunny 11. Thus with ISO 400, 1/500 or 1/250 @ f/11.
It's a rough old approximation but it is aso an excellent illustration that in most of photography, rough old approximations are all you need, at least with pos/neg. As long as you err on the side of generosity, quality losses for over-exposure are very slight, especially with larger formats than 35mm. Grain is bigger with conventional films (though smaller with colour neg and chromogenic) and sharpness is always lower.
Some people expand 'Sunny 16' to include all kinds of other rules of thumb for exposure, which is pure nonsense. Experience enables you to judge exposure extraordinarily well in many situations, but not all, and you can generalize from these rules of thumb, but it ain't anything to do with Sunny 16 unless it's sunny. See
http://www.rogerandfrances.com/subscription/ps basics expoguide.html.
Cheers,
R.