Hops cannot be scaled in the same linear fashion as grain and water. There are lots of complicated reasons why this is so, primarily related to boil dynamics. Also note that you'll have to watch boil-off really, really
I'm afraid I must respectfully disagree with Yooper, tho - you should always pitch the appropriate amount of yeast. That might not mean a full package of yeast. Check the pitching calculator
on Mr Malty to determine the proper pitching rate.