As others have said, "it depends".

The basic concept behind balancing the flash with the ambient light by juggling the f-stop and shutter speed is fairly straightforward once one grasps the idea that shutter speed has little or no effect on the flash contribution to the total exposure. From there, it's really a matter of style, personal preference, and the nature of the ambient light, I think.

Personally, I think it's best to make fill flash as subtle as possible, so it's not obvious that a flash was used. In some ambient-lighting situations, the "right" balance might be a half-stop down on the flash compared to the ambient level, but in others, as much as two stops down. Absent TTL-flash circuitry in the camera, as with your Mamiya C-220, the balance has to be figured manually, of course. Personal experimentation is really the only way to arrive at what pleases you most, I think.