I've used a DLSR to check exposure and lighting on Medium format shoots, but I was wondering about using them for LF nighttime shooting, for example if i set the DSLR to f2.8 at 20 sec, what would I set the LF too, is there a scale factor invloved, also on long exposures the DLSR doesn't suffer Reciprocity failure, whereas the film will, again is there a factor to figured i to account for this?

Throwing it out there, as I'm sure someone here will have the answers, before I go off shooting loads of black film!