bethyl IgG - elisa analysis in excel (Jul/28/2009 )

Hi All,
I am trying to analyse the ELISA data using excel.bethyl lab IgG kit says to use soft max pro and by means of 4-parameter curve fit,they have given the calculated concentration values of the known standaards and it is very close to the known concentration(for eg:500 ng/ml shows abs of 1.826 and the calculated concentration is 501.19)but i am using excel.i used the conc and abs values and with the graph generated,right clicked,added linear regression trendline and displayed the equation which shows
y=0.0006x+0.1285,R2 is 0.998.

with the above formula,i cross checked the standard concentrations using this formula but it gives an entirely different concentration than the known one,so what is wrong or where i should change the analysis.
thanks

If you could paste your data (ie. curve abs & conc, standards abs & conc, your sample abs) then maybe I'll be able to help you.