It's not exactly a formal legal title and there's no 'official' criteria you need to meet.

I'm going to take a critical opposing view if you don't mind (I'm a History PhD Candidate, and don't yet consider myself an historian), but none of this is meant personally or as an attack, I'm just attempting to put forward an opposing perspective. I completely respect your position and absolutely understand and appreciate the respect you are showing towards History writ large by evening asking first.

I do not agree that anyone can call themselves an historian, just as not anyone can call themselves a scientist, a mechanic, or an artist. This doesn't mean that one needs "X" amount of education in any area, but it does mean meeting certain standards. Now I'm going to take only the information you've presented (meaning I have very little specific or additional information which might alter my arguments), and argue why you should not call yourself an historian.

You graduated with a BA, was this an Honours degree? Did you have to present a thesis? Have you submitted any work for peer review? When you use the word "speciality," what do you mean by that? Niall Ferguson specialised in imperial economics, David Bercuson is a military historian specialising on the Canadian Army, etc. One cannot really specialise in an entire field of history as much as one can be interested in it. I "specialise" in the Eisenhower administration's third world foreign policy alongside the input of Richard Nixon, specialities are quite specific.

The biggest red-flag for you calling yourself an historian is that you've not said whether you've actually done any historical work. By this I don't mean written papers in courses or a blog, I mean have you written or done anything that other Historians have looked at, criticised, possibly disagreed with, but still thought was important/novel enough that the wider world or historians and non-historians ought to be exposed to it? This is why I don't consider myself an historian. Admittedly I've yet to submit anything to the process, but other than my MA thesis, I have not written any piece on any topic that has been scrutinised by experts or academics. Still, while I have great enthusiasm for, and expertise in my area of history, I feel like to call myself an historian would be like a med-student who has yet to treat a single patient calling themselves a doctor. Did you see X-Men: First Class? Xavier has a line near the beginning that goes something like "You're not a professor until you teach at least one class." To me, you're not an historian until you've done something which is acknowledged and credible as an historical work.

I'll use the Stephen Ambrose example to close. Stephen Ambrose WAS, for all intents and purposes, a remarkable historian. He certainly and undeniably compiled and presented information and arguments which were praised, and useful, for a great deal of his career. The problem is that Stephen Ambrose was a plagiarist. This means that despite the fact that he, without argument, knew more about Dwight Eisenhower and Richard Nixon than I likely ever will, solely due to the fact that I've never plagiarised, I'm more of an historian than he was.

TL'DR: I guess what I'm getting at is that to me, the title of Historian isn't just about knowledge, enthusiasm, passion, multiple/a single degree or time-spent. Being a legitimate Historian means all of those things, alongside upholding standards of professionalism, credibility, and respect and all that goes along with those standards. While you appear to have the first section, I do not see your proposed project, unless you've gone through legitimate/official channels, as being that of a bona fide military historian. So no, you cannot use Historian as an official title, in my opinion.

Again I mean absolutely no disrespect, and I am basing my comments and arguments on the information you've presented along with my own knowledge, bias, and experiences in the field and in academia in general. I think you should certainly point out your education and experience on your site, but know that while the lay-person might not distinguish, other academics might not consider your site a "legitimate" source. Sort of like Wikipedia (in that everyone, including academics uses it, but it isn't considered a valid source in an academic setting since it lacks a variety of requirements).