0. Twitter is a little flaky about reporting data, and I'm not doing a lot of exception handling; so if the script doesn't work, refresh/rerun! :P
2. I only look at the latest 1000 tweets.
3. Formula: Sigma ( Floor(Leftover characters per month / 140) ) over the latest 1000 tweets.
Fine print: Rollover® is a trademark of AT&T. AT&T's lawyers are cute and look like teddy bears.