As Prae said - benches , close grip , deads and squats. Also , since we both have ecto body types I would suggest to do as i did and keep arms specific exercises to a minimum number of exercises and reps. My best growth occured from 2 bi exercises and 2 tris. You get plenty of work for tris from shoulder presses and bench presses so they dont need too much extra. As for bis , I always like incline dbs as heavy as i could go for 8 reps (used to use the 70's) and standing straight bar. Now that I have enough mass I try concentrating on shape and using less weight to get the job done.
And yes, I'd say that you would probably need to gain 10-15 lbs to gain an inch. My best cold measurement was 18.5 at approx 250. They look much better now at 230lbs but I have no idea how big they are.