fp4 is a beautiful film when processed by someone who knows what they are doing. I would, as others have said, rate it at box speed. I'd also ditch the flash and stick with a straight-forward exposure. To me, these in-camera flashes aren't something worth using, particularly for fine art work. Is there a particular reason that you don't process on your own? it's much much cheaper than having a lab do it, and though it is a pain at first, you eventually learn to love it.
-Austin