The x code of the tippet really depends upon the size of the flies that you will be using. As a general rule of thumb, take the hook size and divide it by 3 or 4. For example, a size 12 dry fly will be most suited with a 3x or 4x tippet.
The rest of the leader will need to be balanced in order to turn over the tippet. For a six weight line, you could probably get away with using as light as 16 pound leader for the butt, and then taper down in individual sections until you get to the tippet.