Copy-number calculation formula? - (Dec/14/2010 )

Hi all,
I know there is a formula to calculate plasmid copy number in a solution. I think I have used it before, but now I just can't find it on the intrernet, so please, if you know this, write it here, many thanks (I want to mix my plasmid with gDNA having finally the transgene as "one copy" in the mix, for positive control)

Ah, finally I found a webpage: http://www.uri.edu/research/gsc/resources/cndna.html