Please look at the following below links which may help you in your analysis:

https://www.longdom.org/open-access/evaluation-of-free-radical-scavenging-activity-of-various-leaf-extracts-2161-1009.1000150.pdf

https://www.alliedacademies.org/articles/in-vitro-antioxidant-activity-and-free-radical-scavenging-potential-of-roots-of-malawian-trichodesma-zeylanicumm-burm-f.pdf

Thanks

At which wavelength did you measure the absorbance? 240 nm?

It appears that DMSO and/or the extract are absorbing so much light at this wavelength that reading blanks (e.g., DMSO only or Extract only) will not be very helpful.

If that is the case, consider using a alternative assay, such as the horseradish peroxidase+phenol red, to change the wavelength of the final reading (610 nm in this case).