This average is called the TAG, or Trailing Average Gross. Every time the starbond is in a movie, the new movie comes into the TAG and an old one drops out.
So, to predict a starbond's next adjust, first you add the box office of the four most recent films and (an estimate of) the box office of the film that will delist next.
Then you divide this total by five, to calculate the TAG, and then divide by a million to get the price that the starbond will adjust to.
Compare the expected adjust price to its current price and decide whether the starbond is a good investment.
That's Starbonds 101. Starbonds 102 is that for starbonds with fewer than 5 movies in the TAG are calculated with different formulas, which are listed in the glossary linked at the bottom of the page under Trailing Average Gross.
Also, I've written some columns on HSXsanity to help new players cope with starbonds. Here are a couple of links that may help.
http://hsxsanity.blogspot.ca/2013/02/how-can-i-calculate-starbond-adjusts.html
http://hsxsanity.blogspot.ca/2013/07/why-is-my-starbond-adjust-prediction.html