Sure, there are a lot of variables but that is one of the reasons I was considering the free to sales ratio over any other ratio. A lot of the factors you are considering would affect unique to free or unique to sales ratios but when someone does sign up the conversion ratio from free to sales has always been pretty much the same for me from site to site.
That said, his traffic is about 50% US, 15% German, 5% French, 5% Canadian, 5% UK, and then smaller portions from a multitude of counties.
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