Are you supplementing it in animal diets or what?. If animal diets, i suggest you start from very low dose like 0.05g, 0.1, 0.2, ........ may be upto 1g/100g. The reason is that plant extract when above 2 or 3 g, causes immunosuppression. Most literature recommended between 0.5-2.0g/100g depending on the model animal and plant extracts used.  

Do toxicity studies According to OECD guidelines. Once established toxicity choose 1/10 th of the highest safe dose for experimental setup. for eg if safe dose comes to be 5000mg/kg the effective dose will be 500mg/kg and so on.

hope this will help 

Do an LD50 on your target animal (presuming this is not human).

The problem arises when an extract does not produce toxicity. You will have to spend time figuring out what you expect the extract to do based on it's chemical composition. Look at other plants with these chemicals and their dose. Extrapolate the dose and identify at what point it begins to act.

Remember Paracelsus said "The dose maketh the poison."