if you are molding a fett helmet... why would would you want to put a seam into the rebound?
my helmet mold is 1 piece... peels on and off like a glove. that way there are no seams to worry about.
i divided the plaster jacked with modeling clay, since that does have to be split.
one thing to remember is that tutorials are really just guides. you have to look at what you are doing vs. what's happening in the tutorial and see what applies.
you can see some pics of my mold making here:
http://stormtrooperguy.com/gallery/thumbnails.php?album=284&page=3