I have been playing Metallica songs (only the rhythm parts) for quite some time. I don't play them at full speed because of the Down picking, but i have been playing and practicing them at 80% speed with down picking and i can't go any further up. I know people tell that one has to practice by increasing the metronome in steps and i have been doing that, but there is no improvement at all (i have been doing this for a month, 5 minutes everyday).

I want to know what is the optimum right hand position for this technique, the pick size to be used(i use .50 pick size), pivot point and closed fist or open hand. I would appreciate if anyone could explain this technique with the Main riff of Creeping Death taken as example.