Amazon sales rank = how many books sold?

The Passive Guy over at the Passive Voice asked some questions, including these:

So, here’s question #1: Is this a fair way of evaluating how well an ebook-only or mostly-ebook publisher performs for authors?

Question #2: While we’re discussing Amazon Best Sellers Rank, has anyone seen any credible discussions concerning how sales rank translates into dollars for indie authors? If you’re at 20,000 with a $2.99 ebook, are you making $500 per month from that book?

Or is Amazon’s algorithm too volatile to make it a useful gauge of the dollars coming in the door?

Question #3: Everybody knows that lots of ebooks have experienced sales spikes immediately after Christmas as new ereaders and tablets are packed with ebooks. What about other sales patterns? For example, do sales go up on the weekend when many people have more leisure time to read?

I've never had a publisher, so I don’t have any information to answer the first question. However, I do have 18 months of daily sales and rank data to address the second two questions.

So, first off, how many books sold does a given Amazon sales rank indicate?

Unfortunately, due to the hourly fluctuation of Amazon sales rank, we can't pin it down precisely. But I have enough data to make some generalizations for ranks between #333 and #100,000.

All the following data is for Amazon US sales rank only. Also, Amazon changes the ranking algorithms, so my results from six months or a year ago might not equate to what is happening today. I don't sell enough books in other markets or through B&N to have a sufficient data set.

Here's a chart of my Amazon Sales rank for my best selling novel, Dead Dwarves Don'tDance:

And here's a spreadsheet with sales and rank data for each month:

What can we determine from this data? Here's a table with some guesses:

Again, this is based on historical data over the last 18 months. Other author's results might be different.

Also, the estimates for months that showed a lot of fluctuation aren't as reliable as my months where my rank range was smaller.

As for Passive Guy's 3^rd question about sales patterns, I have not detected any daily patterns. The only yearly pattern I've seen is better sales around Christmas and a slump in Summer. I have not been able to identify any sales increases from any paid or free advertising, such as reviews, articles, and such.

Do my numbers align with yours? Do you have any other information about rank to sales and patterns?  Comment below if you do!

PS: And don't forget my Kickstarter project. If you want a signed poster of the cover of Dead Dwarves Don't Dance, please check it out. You'll be helping me commission another great cover for the sequel.