For my pistols, I like 2.9 grains of Clays with 125 grain bullets.
Since the Marlins like a little longer bullet, I used 3.2 grains of Clays with a 158 grain bullet. I loaded them in 38 Special brass but crimped the bullet at the top of the lube groove. That way the cartridge is almost as long as a 357 Mag case, but with the crimp at the top of the lube groove, the bullet cannot be pushed back in. Worked very realiably in my Marlin. Or you can just load with 357 Mag brass, but since 38's are so cheap and plentiful.
(Some Marlins will work great with the regular 38 Special length, but I just wanted to be sure of reliability so loaded longer.)