If you can get your hands on some Vihta Vuori N320, the book max for plated 158s is 4.8gr; while it's not exactly what you had in mind, I will say this: it's the highest performance .38 Special load you can make, and the cleanest dang thing you'll ever shoot in your gun. Plus, it makes USPSA/IDPA "minor" by a mile in a four-inch gun, which most SAAMI-level .38 loads might not.
For Berry's plateds in general, I have found that powder charge weights tend to be in the middle of the equivalent lead or jacketed, and often tending towards the jacketed end. That may be at variance with the manufacturer's advice, which usually says use lead charges.