Skeptoid, passability is not, not, not, not a requirement for using the Women's. I have a lot of MtF friends who don't pass, but who use the Women's all the time.
You are a woman. The women's room is where we go. It just is. You have just as much right there as any woman. Anyone who gives you a hard time about using the restroom, either because of their opinion of your gender (which doesn't count) or because of any other equally irrational and unreasonable basis, is in the wrong.
When we are presenting as women, passable or not (it doesn't matter), the Woman's room is the place for us to "go".