Now that we've been tracking feeds with FeedBurner for quite a while, the big question is, "How many total subscribers do I actually have?"  Don't get me wrong, FeedBurner is great, but I'm astounded that they haven't addressed this question.  It's all over the Google FeedBurner group.  It's nice to see the daily number of subscribers, but seriously, what's the total?  Here's my attempt at estimating the total number of subscribers to your feed with FeedBurner that I posted in my response to the group:

1)  Take the average subscribers over the last 7 day, or last 30 days depending on how often you post.

2)  Multiply the average subscribers for this time period by the number of days in this time period.

3)  Divide the number from step 2) by the total number of posts in the same time period

My assumption is that most subscribers will read each post one time. Those subscribers that read some posts more than once will cause this number to be a little high.  This depends a lot on your content too. This also assumes that when they re-read they have to access your feed again, but this differs for various feed readers.  I would guess that web based feed readers won't cache all the feeds.  However, it looks like Google Reader does because I've tried modifying a feed but the changes don't appear.  Feed readers could also cache a feed based on how many subscribers there are according to them by using a lower-limit of subscribers requirement.  For example, Google Reader may not cache a feed unless there are over 50 subscribers to that feed with their reader (this is not necessarily true, just an example).  In reality, this calculation may be more accurate if most feed readers (web-based or otherwise) do cache feeds.