Method 1: Using the sodium nitroprusside standard curve, convert the obtained absorbance to sodium nitroprusside equivalent as milligrams per gram of dry material. To begin, draw the standard graph and obtain the equation. Then substitute your sample value for Y and find the X. Then convert it to its sodium nitroprusside equivalent as milligrams per gram of dry material.
Method 2: % scavenging activity = [(Acontrol − Atest or Astd )/Acontrol ] * 100, by substituting for this equation find the % value for your sample and std. It is preferable to create concentration series for both the sample and the standard, then obtain % values for each concentration and get IC50 values from that. Then you can compare the values.