You can comparison between your results with other results for same plants.

I agree with Abdelrazzaq, check the literature and compare your DPPH data with other similar studies. In my opinion 300-400 g/L which is the same as 300-400 mg/mL is way too high for crude extracts...ideal bioactivity should be observed at less than 1 mg/mL

I think there is problem with your calibration curves. The other problem arise when you are trying a narrow range of your substrate (plant extracts in this case). These problem can be easily observed during experiments like in the first case there would slight decrease in absorbance whereas in the second case, whole of the DPPH would colorize simultaneously. In order to avoid this, you should try wide range of extract concentrations 2 microgram/ml to 5000 microgram/mL and see the range of caliberation curves, if their slopes are linearly increases with smaller concentration and remains constant with larger one, the assay is okay, otherwise you have to choose another range of plant extracts. I have always obtained good result for 1mg/L DPPH solution

for more detail you can see 

https://www.researchgate.net/publication/263424616_In_vitro_Synergism_of_Antimutagenic_and_Antioxidant_Activities_of_Phoenix_dactylifera_Fruit

Best of luck. 

Article In vitro Synergism of Antimutagenic and Antioxidant Activiti...