i my own experience the doses can vary significantly which is very logical when you have in mind that the extracts are the mixtures of different compounds. You need to determine the LD50 before you start to treat your groups for the possible anti-diabetic activity. Give three doses of extract eg. 100, 500 and 1000 and observe the animals for few days, or try finding toxicity studies for your plant extract and compere the composition of extracts and try to draw some conclusions from them regarding the toxic doses.