Anyway, the main concept behind F/T is how one makes decisions. That was my tool for typing each character.
Jack has shown numerous times that he mostly makes decision based on emotions and people, rather than task. When someone is in danger, he will leave everything and try to help him, not necessarily because he's a doctor, but because he wants to help. Even though he knows that inviting other people would benefit his ways, he wouldn't do that because he may put them at risk.
Locke has a lot of F traits, indeed. That's why i typed him as a borderline T. But the F traits are mainly because he's ilogical at times, with all the mysterious sutff(although it's unfair to judge him about that, that island is really special and "real world rules" do not apply there). The T traits about him are more dominant, though. He definitely makes decisions based on task, he doesn't give a damn about the others as long as he gets what he wants and he doesn't appreciate people getting in his way.