After I wrote my post, I found this post on New Media Lab that makes the same point I did. It also has a neat  graph of “distribution of Tweet length.”

Original post:

Your updates on Twitter can be a maximum of 140 characters.

However, your ideal Tweet length should be a little shorter.

If possible, your Twitter updates should have no more characters than the following formula allows:

135 - [length of your username]

Why?

So other Twitter users can re-Tweet your updates without having to edit them. If a fellow Twitter user has to edit you, he may change your message, even if only slightly, or he may simply decide not to re-Tweet you at all.

Where does the formula come from?

As you already know, 140 is the maximum update length Twitter allows. Take off another five to accommodate the R, T, @ and two spaces needed for the “RT @[username] [Tweet]” syntax that is the accepted standard for re-Tweeting. The length of the username determines how much space you have for the Tweet.

My Twitter username (mbusse) is six characters, so following this rule I should keep my Tweets at a maximum 129 characters each.