I guessed when doing mine: people.tribe.net/btd/photo...47b939bbda
I knew to do a B setting, and having done some long exposures before (of night time street scenes), I guessed 30 seconds, opened the shutter and counted, and hoped a bolt would happen.
It was a particularly intense storm, and I got one (as you can see) - after a few attempts.

The camera was a non-name brand from the 80's, no electronics, no auto-anything. (Cosina CT-1A if you really want to know).
The lens was a 35mm-200mm Vivitar.
Film was probably the cheap band of ASA 100 - but might have been 400. Sorry it was 21 years ago!