The webpage http://www.novelrank.com/ provides a very neat simple way to track the sales rank of any book on Amazon. This service is completely free.
The sales rank is computed from the sales rate. The more a book sells per day, the lower the rank will be. Here’s an approximate formula, taken from: http://www.edwardwrobertson.com/2013/02/a-quick-way-to-calculate-amazon-sales.html.
100,000 / rank = sales per day
So if a book is on rank 50,000, it sells about twice a day. As far as I know, a borrow counts as a sale and a free download as one third of a sale.
I use novelrank to track my ebooks. This is what the output looks like (launch of “Great Formulas Explained”):
Indeed a neat tool to see how a book is performing. Note that the tracking starts on the day you add it, dates before that are not shown.
As you can see, during the period when no sale is made the sales rank increases more or less linearly at about # 50,000 per day. The average rank during this time can be calculated by the formula: final minus initial rank divided by 2. When a sale is made, the rank makes a discontinuous jump to a lower value.