If you go on Hodgdon's data site, pull up 38 spl and 357 mag with the same weight Hornady FTX bullet, same powder, and with the same powder charge, both measured from a 4" barrel, the 357 shows more fps even though it has greater case volume. The gun can mean as much as the cartridge.
boatbum101 does have a good general rule to follow. Unless your at max load, bumping up the powder charge .2 grains should compensate for the extra case volume.