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,