Fill flash math.

I was wondering about how fill flash affects the existing light. Of course
it will take the shadows and lighten them up a bit, but what about the parts
of the picture that are correctly exposed by the existing light? I mean the
light emanating from the flash (attached to the hot shoe) does not just go
to the shadows. Wouldn't the areas not in shadow then be overexposed by a
half stop? My fill flash photographs (from print film) have been coming out
nicely but this conundrum has me scratching my head. On second thought,
should I be using some ratio method to compensate for the existing light
exposure or is that not how it's done? I'm using a Minolta Maxxum 7 with a
3600HS flash unit.