What's your motivation for using Pan F with flash at a party? Frankly, does not make a lot of sense to me...
-) Pan F is a pretty contrasty (at least in my experience) film, and with contrasty on-camera flash, this is gonna get worse...
-) Unless you have a VERY powerful flash (read: Metz 60, or something similar), your flash won't reach very far, and bouncing the flash (a prerequisite for somewhat natural looking lighting) will be out of the question.
-) Pan F is usually recommended for large, grainless prints from 35mm films - do you really want to do 50x60cm prints from party snapshots?
I'd rather do it the other way round - use Neopan 1600 (rated at around 800 to 1000) with a slow shutter-speed (1/30 to 1/60) and large aperture (f2.8ish) to capture some ambient light, and use the flash in addition, to prevent motion blur and assure the light is enough....
Totally different concept, though...
So, what is your motivation for PanF?
Roman