yes, what uncle catchlights said, use STOPS to determine.
1 stop roughly doubles, so if you look at his post,
1: ISO3200 , f11, 30 seconds
when you HALF the ISO, that is one stop less, you have compensate by doubling exposure time to 60 seconds, the next line
2: ISO1600, f11, 60 seconds
similarly, for f-stops, the same applies. multiplying by 2 is really easy.
as to how to determine, just shoot at very high iso, if the histogram looks right, then adjust accordingly.
if the light is changing, then best to compensate a little, for example, if it is transiting into darkness, remember to expose more. long exposures are no joke, you don't want to wait 32 minutes to get nothing out of your wasted time, just because you didn't know what you were doing.