There is also an "lazy man method", if your camera (or shutter) have intervalometer ;-)
Then recipe is -> use settings as above (but set time to 30" instead of B), set 30" intervals & something like 100 frames to take.
Go watch a movie or something.
Come back when the storm is over, import images to PS as layers, set blending mode as "lighten" for all layers except background, enjoy all the lightnings in one picture ;-)