How advances are calculated!

Too lazy to read the Guardian article below?

Here's the best/worst part:

Williams punched Carpenter's forecast into an in-house spreadsheet that uses the estimate to compute a suggested advance to be paid to the author. "The problem is that it almost always comes up with a figure of about £4,000, so you have to ignore it," says Williams. By Monday morning, she had made a low five-figure offer.