I could live with around 75% or so. That's what I told myself at the beginning of transition, was that I'd be happy as long as an average rational person could look at me and gender me female. That way I'd feel comfortable correcting the minority who I didn't pass to.
And yeah, I was pretty damned happy back in February when I was around that 75% mark. Hearing "sir" still hurt on the couple days a week it happened, but most people I'd never met before were gendering me female, and I was pretty damned happy about that, and went full-time very shortly thereafter. (After getting fired from my job.
) And I think I could have been perfectly happy if that was as far as I ever got. I'm glad I've matured mentally since then, but physically I felt pretty damned good at the time, and so excited that I was starting to pass more often than not, and I do think that with enough time, I could have found a way to be perfectly happy there.
I really don't think there's a set mark of "passing" that can automatically make someone happy, though. It's all mental. Some of the happiest trans women I know are the ones who are only moderately-passable, but forgive themselves for it and accept themselves as women anyway. And several of the most miserable trans people I know are ones who are absolutely gorgeous and passable, but are still hung up on not being cis. It's all a matter of forgiving yourself for your "flaws," (and we ALL have them in our own minds, even if nobody else can see them,) and deciding that we're not going to hate ourselves for them. That's the real key to being happy, is just being your authentic self without shame, and with a strong self-conviction that your identity is just as valid as anyone else's, whether you're passable or not.
Maybe that's a bit hypocritical for me to say, because I put myself through a mental hell of self-criticism before I started passing most of the time, and I was waiting to pass before I started feeling validated and accepting myself, but it did take that acceptance for me to move from a place where I was miserable and constantly hung up on the last 10% of the time that I wasn't passing, to being happy for the 90% that I was. I decided to quit being jealous of my 100%-passable friends.
I've heard a lot of trans women say very proudly "I'd rather pass 30% of the time than never," because they realize that they're happier than they were pre-transition, and take that as being enough for them. That's the ultimate realization, is that nobody is more "authentically female" than anyone else, and that passing is really pretty arbitrary, all things considered. It's basically just organizing people into a hierarchy based on a genetic lottery of age and ancestry and susceptibility to HRT. Which seriously isn't fair. I refuse to tell myself that somehow my identity is less valid just because my mom's side of the family has a history of broad shoulders, pointy Italian noses, and MPB.
