At Clickz.com, they claim to have come up with a formula and while they do not reveal the formula, they do discuss a few things to think about.
Calculate Your Online Conversion Rate
Sponsored by WebTrends
By Bryan Eisenberg August 13, 2004
Mal Watlington, president of specialty consulting group, City Square Consulting, offers a blog on the topic of online conversion:Online Conversion & Beyond discussing a variety of topics on marketing.
Put the "Find" Back in Search To Increase Online Conversion Rates
By Moshe Ofer
November 1, 2005
The Art of Online Conversion: Four Steps From Interest to Acquisition
by Brian Dempsey June 27, 2006