I also disagree with the person stating that America is 75% Christian. It really depends on what you define as a Christian. I define a Christian as a person who is trusting Jesus only for their salvation and sanctification and are following His teachings. The 75% who label themselves "Christians" are no more Christians than I am a fern. If I had to guess (and it's only a guess), I would say less than 6-7% of us are actually Christians.
You're right it depends on how you define Christian, but some definitions are more useful than others. Why don't you just define Christian as "Someone who believes exactly as I do", making you the only Christian in America?
To the rest of the world, who want to communicate with commonly understood definitions, the US is roughly 75% Christian.