according to Uncle Ben (and i see no reason to doubt him) you should top at 5 to 6 nodes (and you are right that is pair of leaves = 1 node) then top just above the second node or second set of stems. that will leave your plant with 4 steams and each will be pushed up to make a new top. any later and most agree you will get undersized top buds that cant really be called colas. if you wait till later to top the growing potential will be spread out over more then just the 4 branches and will not have what it take to produce colas. - this is all from reading not from actual experience, to get the info first hand got to uncle bens post stickied in the advanced growing tech)