Well, it was only an approximation. Like most people I spend a lot of time thinking about neutral things also. It's possible to do a sort of survey of yourself when you're at home with nothing much else to do for an hour.
Get a large sheet of paper and write some headings across the top with topics you think you think about, leaving some space for extra headings. Then just allow yourself to feel at ease and put a mark under each heading every time your mind goes to a new topic.
You can get more insight about yourself if you also put a plus/minus/zero next to each mark depending whether the thought was pleasant/unpleasant/neutral. When I first did it I was very surprised at the result: a lot of my time I had angry thoughts about other people. Not so much these days though, I'm glad to say (hey, a happy thought!).