Scrumpets! I thought I was the only one who counted the letters in sentences!
Another peculiarity in my habit is to have all the letters come out even. I insert a comma, period or exclamation mark if I come up with an odd count. All this I do in my head, of course. I've never thought of this quirk as a drawback; in fact, it has greatly enhanced my spelling and other cognitive abilities.
It would be interesting to have an expert explain this letter-counting phenomenon in psychological terms. I’d like to know in which part of the brain this little quirk lies, and how it came to be.