I think the general rule is, if strangers are gendering you female, use the women's room. If strangers are gendering you male, use the men's room.
And I don't mean completely... I just mean a reasonable majority of the time, where you can safely say to yourself that an average rational person looking at you is probably going to be able to see you as a member of your identity gender. (And even if you're not getting properly gendered, you still have a right to use the bathroom of the gender you're presenting as. Get a carry letter or something if you have to, and nobody has the legal right to tell you that you don't belong in there.)
Another good indicator would be that if you're getting stared at while using the bathroom, or if people are giving you weird looks, it's getting about time to move to the other one. (Everyone goes through an androgynous phase, though, so there's a chance you'll get stared at no matter which bathroom you go in, so take your choice.)
Back in February, I was starting to get gendered female by strangers at work rather often. (Once or twice a day at least, sometimes more, while pretty much the only people still gendering me male were those who knew me already. And there were a lot of people who were asking me what my name was before gendering me, which was another big affirmation that I'd reached the andro phase where people couldn't necessarily tell my gender just by appearances anymore.) And there were a few times where when I went into the men's room at work, people were staring at me. And there was even one time where one of the guys at the urinal lurched back a bit and hid himself when he saw me coming in.
One night, I finally reached a point where I said "you know what? When I pull my hair into a ponytail and wear dangly earrings, I don't see any way that someone could gender me male." And so I decided to try using the women's room.
Nobody stared at me. I was nervous as hell (as probably everyone is when using the women's room for the first time,) but people pretty much just ignored me. And I've been using it ever since, and it's been 4.5 months since then, and still nobody's given me even a sideways glance.
So seriously. If lots of strangers are gendering you female, and if people are asking your name before gendering you, that's probably a good indication that you can start thinking about using the women's room.
I personally waited until I was really confident that I was passable, but again, that's not a requirement, that's just because I'm an overthinker and a worrywart. But hey, it worked.