making your own leader

I would like to start making my own leader. How much will using a
double nail knot affect things? I would probably use flourocarbon (what
are the best? Is berkely vanish good? It is fairly cheap at $9 for 250
yards. So assuming that a double nail not is ok, what I am looking for
as an answer would be (for instance)
"to get a 6 pound test 8 foot leader, use 3 feet of 10 pound, 3 feet of
8 pound leader, and 2 feet of 6 pound leader.