4 parameter logistic model for fitting PCR data

[requiem31]

I have a question about the logistic fit they used on this paper: http://online.liebertpub.com/doi/pdf/10.1089/cmb.2005.12.1047. Its PDF page 5, or journal page 1051. They define all the variables and then say x0 and b define the shape of the model. If I am fitting 30 cycles of PCR data how am I suppose to know what to use for x0 and b? -Thanks

 

[mfb]

If I am fitting 30 cycles of PCR data how am I suppose to know what to use for x0 and b?

Use your fit results? Those results should be the unknown constants.