You can use the extract directly with the ratio of 1:1 of your extract in aquades and DPPH in ethanol. For such purpose take baseline of 1:1 aquades and ethanol to determine scavenging activity using UV-Vis spectrophotometry .
You can use the extract by concentrating reduce the volume of aquades in extract and dissolve in dmso at the same time DPPH disolve in the ethanol, do scavenging activity by UV-Vis spectrophotometry .