Learn this secret formula from a Pro during my photography class.
Exposure time = appature. eg. if appature is f5.6, exposure will be somewhere there, say 6 sec. Formula applicable to ASA 100 film. If film speed is ASA 200, reduce you exposure by half.
Also note that any appature setting above f11, formula become inaccurate because of the nature of the film.
But just to be on the safe side, bracket your shots.