Ammo
I have a 640 and it groups pretty much right above the front sight with 158 gr. bullets. But the groups are not tiny, so it might be off 1-2 inches at 25 yards, who can say. Maybe different ammo would group in a different place.
Might be interesting to see if the groups move any laterally with different bullet weights. But with that short sight radius it might exaggerate any errors or the fact that if the sights are bare metal(stainless) finish, the sunlight angle can affect your sighting depending on which side of the front sight it is shining. Might black out the front sight and see if that does different. Otherwise maybe you could get some laser grips and set those up to hit right on line at your optimum range of 12 or 25 yds. Otherwise fixed sights can be altered for windage a little, but might be alot of gunsmith work.