Forget any "formula" for 38 Specials in 357 Magnum brass. Yes the pressures will be lower in the longer brass, but as with any reloading using new/different components, use starting loads and do a load work up, and I've even used 2.9 gr of Bullseye under a 150 gr DEWC in 357 Magnum brass. Never stuck a bullet in my 6" 357. I have used hundreds (thousands) of Special loads in Magnum brass, both 38/357 and 44 Special/44 Magnum. From mild to wild, just do a load work up and review, watch for problems...

I've gone both ways Special loads in Magnum brass and Magnum loads in Special brass. Just think about what you're doing, understand more powder means more pressure and K.I.S.S.

The major "problem" using rifle primers instead of standard primers increase in pressure. Rifle primers are often the same as Magnum primers and are "hotter" so again, start low and work up. Also rifle primer cups are "harder" and light firing pin strikes may cause misfires, but I've never had that problem in a revolver...