As someone who often quotes the desire to pass 100 percent, allow me to share my perspective. I don't think there is anything like 100 percent. I've always made my goal to strive as close to that number with the understanding that perfection is never possible. Not just with transition but with life. In my case, I'm really trying to slowly overcome my deeply rooted social phobia and learn how to be happy with my situation and with who I am as a person. Passing is simply a way to make the transition phase easier and allow me to feel more comfort in my presentation which helps me get to where I want to be. Let's not deny that passing privilege exists. It does. Still, I imagine the more I grow as a person and start progressing further into my transition these concerns will be behind me, as it seems to happen for many transwomen. It's often common for people to live in fear before taking those steps. I'm simply taking the steps I'm able to take at each point and then work on perparing myself to take the next one. And I'm a much different person from where I was a year ago. I've grown in many ways and have taken on things I never thought I could. Believe me, I feel that I'm a better person both emotionally and physically. Sure, there have been misteps, and a lot of fears, and concerns, but not so much doubt. More like "how can I do this" or "what does it take for me to see this through". In the end, I really don't doubt the happiness factor or ability to get there. It's just finding my way and learning what it'll take for me to overcome these very real hurdles that I set up for myself and ones the world gives me. So, 100 percent is more of a figurative statement, but it is indicative of the direction of the result I hope for. In any case, I realize that happiness and self comfort/confidence comes from within. Passing or not, this is something that needs to be developed and I believe my transition will ultimately aid in me acquiring these qualities. Passing only makes it easier or more tolerable to endure the rough initial stages that need to be taken on before confidence can be grown.
I can't speak for others, but that's how I see my own situation in the context of the "100 percent" statements.