Well, you're gonna need a doc to come up with a nexus statement. C&P doc may draw the connection, but my understanding is most of them are RNs, this type of story would best be explained by a neuro who is up to date on research, VA or civilian. A doc needs to say it is as likely as not, 50% likelihood (or a better statement), that you migraines were caused by your tinnitus, or that your migraines were in an early stage while in service. Some people think a VA doc is taken more seriously, but I just think they write things up better to what a rater can understand. Any doc that will support your idea as realistic is good, but they need to back it up with some sort of literature, can't just pull any old idea out of the air. Part of a doc's job is to be up to date on research out there, so they should know if there is a valid backing to the story.
Like I said, I have heard of migraines taking awhile to develop into prostrating. I have heard of tinnitus being a possible sign of migraines, tinnitus is normally secondary to hearing loss, but you could say tinnitus is secondary to migraines. So since they know the tinnitus started in service, and you can establish that your tinnitus and migraines are connected now, they should be able to say the migraines started at the same time as the tinnitus. I could understand a case for migraines starting in service, despite a lack of medical treatment for them while in service.
I have not heard of tinnitus causing migraines, which is what secondary connection means. Tinnitus is the #1 claim out there I think, many have it without migraines. Of course many have migraines without tinnitus, but aura is a common relationship to migraines, though normally visual auras. Really though, its how a doc can justify the nexus that matters.