Hi gang,
I'm shooting a wedding for a friend this weekend (nothing like waiting until the last minute to ask a question) and the ceremony is right after dusk outdoors. There are going to be fires burning in the background so there will be a lot interesting light to capture in the background. I wanted to shoot with flash but leave the shutter open for a bit to capture some ambient light. I like that effect of the initial flash plus the blurry ambient light motion (know what I mean?). I just don't want that flat flash look. I'm using a Nikon SB-800 flash on auto (not TTL) with my Nikon F3 with Fuji NPS160 (open to other film choices). My knowledge of the SB-800 is very minimal since I'm borrowing it. How can I figure out how much compensation to give with the longer shutter speeds? I am going to go out to the location when the weather clears up and run a test roll at the exact time the ceremony will be. Any suggestions before I start testing? I'm thinking shutter speeds around 1/8 to get some blur. I have a Sekonic L-328 flash meter that I'll use be using for the tests.

Obviously, this isn't your usual wedding photography because they aren't your usual couple.

Thanks in advance for your suggestions,
Ara