I struggled with this for a bit, but then I got some good advice.
I'd been to see a therapist, and I'd been on hormones for a couple weeks when I came out to them. I had planned to wait until i was far enough down the rabbit hole that they'd believe I was serious (6+ months?). The advice I received from a friend though made me look at it from their perspective. If my own child was struggling with something as monumental as this, I'd want to know about it. I would also want to be there for the journey, not just have the finished product unveiled to me at some point down the road. So that's what I did. I went ahead and told them.
I was naive though to think they'd have some inkling of understanding, that something from my past would make them say, "hmm, you're possibly right." Instead, I got showered with a $h1t-storm of hate and rejection and bigotry and denial. It wouldn't have been any different if I'd told them any later either.
Not sure if that helps, but basically, your parents are going to take it however they're going to take it, now, or later, so it really doesn't matter when you tell them. Just make sure you're serious and committed and prepare for the worst, or their opinions could shake the foundation you've been so carefully laying recently, but always hope for the best in people. It's a happier way to live.