You can use 6.8gr to 7.0gr of HS-6 to get the velocity you're looking for in a .38 Special case BUT, those loads are most probably in the +P pressure range. Considering you're shooting them in a .357 Magnum revolver that won't matter. I would start with a charge of 6.4gr which is a high standard pressure load and work up until you get to the velocity you're looking for. HS-6 is a great powder for lead bullets and usually produces very accurate ammo. (use a magnum primer)

__________________
Freedom is never free!!
SWCA #3437