How to use Real-time PCR to detect some gene copy numbe in plant genome? - (Jul/21/2009 )

Hi, every one , I am a new comer on BioForum.
Today, I have some of questions to ask you for help!
I want to detect the copy number of a gene in plant genome or transgenic plant by Real-time PCR ! I have read some of papers, but I don't understand they how to perform. In those papers, they use standard curve to analysis the copy number. they add the plasmid of known copy number to unknown samples. But I don't know that they how to gain copy number of the unknown samples, such as single copy. In my opinion, the results only represent the concentration of unknown samples by this method, but how to transform to copy number.

Is there anybody can help me or give me some suggestion? Best regards!