I'm pretty sure I solved the problem with URLs included in Little Pork Chop tweetstorms.
Then I had to call Twitter to find out how long a shortened URL would be. They told me.
I had LPC do the math. And I'm pretty sure now it gets it right, and if you include URLs in your tweets, no matter how you specify them, we'll figure it out and compute the length of each tweet correctly.
But I have to put in the usual disclaimers. When you think you have a problem solved, it only serves to uncover another problem. So my fingers are crossed.
Thanks again to William Mougayar for helping me track down this issue.