Your flash is automatic, right? Let it pick its own exposure. Given you have so little ambient, you will likely end up with flash-dominated images if your exposures are short enough (1/60) to be steady with people in them. An ISO400 film would be a much better choice here because the ambient would be only one stop down at f/2.8 1/60. At 64, the ambient won't even register unless you drag the shutter (long exposure plus flash).
An event is not the time to be learning how your equipment works.