That's a good question. Typically, fixed-sighted .38s and .357s will have been roughly sighted to be a bit (2"-3") above POA with 158 gr. bullets at 25 yards, at least theoretically. In practice, I've found much deviation in this regard, and have also found several .38s and .357s that will put 125gr. bullets spot on POA at 25. The 2 1/8" barrel of the 640-1 will show less difference between POI with different bullet weights than will longer barrels, and at ranges under 15 yards, you won't usually see a whole lot of difference between different bullet weights, unless you start talking about slow 180 grainers vs. fast 110s. I have a Colt Cobra that shoots 158gr. bullets WAY high at even 7 yards, so I keep Cor-Bon 110gr. DPX or 100gr. Pow'rBall in it, both of which still are a tad high. I had to file down the front sight on my 681+ Performance Center Quad-Port 3-incher for hot 125gr. loads, because the ports work well and hold the muzzle down. All the rambling above leads to this: You'll just have to shoot some different loads and see what comes closest to what you want.
