I would just install pressure compensating drip emitters designed for gravity fed systems where you want them (they will all emit at the same rate as long as you hav the source high enough and don't kill the pressure with excessive friction loss due to long runs. ) 1/2" poly tubing and emitters are cheap and they take the guesswork out of the job. .
You may find this link very helpful
http://www.dripirrigation.com/drip_i...hp?pgv=Gravity