(posted 8813 days ago)This has also been discussed on the photo.net Q&A forum, you can find a thread her e, while thi s thread discusses how to use fill-in flash without ruining the atmosphere (but since you are doing B&W it might be less relevant to you).
If you can't do Polaroids, then use a light meter. Bracket widely: film is cheap, models are not.
Sorry, I have no clue what a Holga is...