Dear Sir. Concerning your issue about the calculation of FRAP value of plant extract. The ability to reduce ferric ions was measured using the method described by Benzie and Strain. The FRAP reagent was generated by mixing 300 mM sodium acetate buffer (pH 3.6), 10.0 mM (tripyridyl triazine) TPTZ solution and 20.0 mM FeCl3.6H2O solution in a ratio of 10:1:1 in volume. Samples at different concentrations (100,200,300,400 and 500 μg/ml) was then added to 3 ml of FRAP reagent and the reaction mixture was incubated at 37 °C for 30 min. The increase in absorbance at 593 nm was measured. Fresh working solutions of FeSO4 were used for calibration. The antioxidant capacity based on the ability to reduce ferric ions of sample was calculated from the linear calibration curve and expressed as mmol FeSO4 equivalents per gram of sample (DW). I think the following below links and the attached file may help you in your analysis:

http://www.ajpcr.com/Vol5Issue4/1256.pdf

http://www.academicjournals.org/journal/JMPR/article-full-text-pdf/75864D741228

Thanks