I think typically, JP writes most DT songs himself, and then brings the others in to fill out and expand on his parts. We'll never have confirmation of this, because JP is the type of humble guy who's not going to come out and "that was ALL ME" like MP would have. Ultimately, though, I think JP is being generous by letting the music of the songs be credited to "Dream Theater". I think it's probably 70% JP, 15% JR 6% JM, 5% MM, and maybe 4% JLB.