Honestly, I'd tell them that I was a woman even before transition.
Going into transition it was so easy to think that somehow I had to "become" a woman, had to earn it somehow... either with passing in public, or being full-time, or being on HRT for a certain amount of time, or "male fail"ing, or surgery, some arbitraty big moment where I'd be complete and could earn the right to call myself female now.
But when I finally did go full-time, I realized something... there was nothing to earn. There was no moment where I magically switched from male to female. I was still exactly the same person. I acted the same, wore the clothes that I wanted, made the same jokes, liked the exact same things. The only difference is that I wasn't constantly plagued by a feeling of "wrongness" anymore, while acting that way and doing those things. And I wasn't constantly holding myself back anymore either, stopping myself from doing the things that I really wanted to do, acting the way that I really wanted to act, because "it would be wrong for a guy to do that."
That's the thing. I was always a woman. The problem was that I wasn't free to be the person that I knew I was, because society kept telling me that it was impossible for me to be female because of my body. But I've always known it. Ever since puberty started, I've known that my body wasn't supposed to be going through a male puberty, and that I hated being treated like a guy, and knew that being a girl would be what felt "right." I didn't understand it at the time, I thought I was just a boy who wished that I was a girl, and being a girl was something that I'd have to earn, or worse, that I could never be one at all. But the truth is, it was my mind telling me that I really was a girl, and that the problem was that I wasn't free to be who I knew I should be. I realized that the only thing that was ever wrong was the society that told me that I couldn't be a girl, because they don't understand that gender is in the head, not the body or the genitals. I was never a man. I was just a woman with a penis who went through the wrong puberty against my will because I didn't know there was any other option at the time, and didn't know that it was perfectly okay to be female even though I wasn't born with a vagina.