I don't know specifically about the 640 Pro, but S&W zeroes their J-frames for 158gr loads at 7 yards.
Most people shooting J-frames report consistent POA/POI with 158gr loads.
I haven't found much of a difference between 130-135gr loads and 158gr loads, but most of my shooting is inside 10 yards.
I don't know if there's much, if any, difference between .38 Special and .357 Magnum.
It might help if you could give us more details, like make/caliber/weight/etc. of the ammo you've been using, what distances you've been shooting, and what differences you're seeing between each type of ammo.