Counting Number of Readers for a Feed

There are various services or tools such as Feedburner, which estimate the
sum of readers for a given feed. However, they need the feed to be hosted
remotely, re-directed, or whatever else they may do these days.

Given that feeds are requested by:

* Crawlers
* Ping services
* Live bookmarks
* Requested periodically by on-line (public) aggregators
* RSS-forward-by-mail (polling)
* (Human) Readers who cycle daily/weekly/...
* Readers who poll for RSS changes in the background

Is there some kind of a 'golden rule' for the calculation and derivation of
readership? Knowing the number of hits for a given feed is far from
informative. Surely if the statistical sample (number of users) is high
enough, the numbers can be approximated rather well. That said, the theme
of the site would be a factor too, but to what extent?

Any answers or opinion on the matter?

Cheers,

Roy