You know it after testing the effect of different concentration of the plant extract

You can know it by prepering different concentrations from plant extract and testing its activity against the amoebia.. the concentration that give the largest inhibition zone its the appropriate one .. good luck

Of course not knowe before testing the effect of different concentration of the plant extract

Of course not knowe before testing the effect of different concentration

it will be decide after testing the effect of different concentration of the plant extract how much potentail of the crude extact

It depends on what you are testing in the bio assay .

The potent drug candidate must have small IC50 value, MIC or other units. For example, crude extract with a cytotoxic effect, the IC50 value below than 40 ug/ml categorized as strong activities. For antibacterial of single compound, the MIC values at one digit concentration said a strong activity and below than 100 ug/ml is reasonable to said a strong activities. Potent antibiotic must have MIC values below than 1 ug/ml. So its depend on your target area, you should set up the maximum concentration of extract will be tested below than 100 ug/ml if you wan to get an extract with a strong activity, and of course the possibility to get the active extract will reduced. But once you get an active extract, you will get a potent extract.

Usually you have different concentrations to start with. I.e you can do serial 2-fold dilutions of the plant extract maybe starting from 500 ug/ml so you can do 500, 250, 125,.... up to 0 which is your control which does not have the extract. This is just a screening process and for the most potent ones you then do MIC and MBC.

It is usually after testing the activity of different concentration of the extract against the test organism. Then finding out minimum inhibitory concentration of the extract.

First of all, you need to find a similar study already published by previous researchers and follow the procedures they employed. Secondly, you need to try different concentrations of your extract to see whether you will achieve different/better results

For the invitro study, test different concentrations of the plant extract, for the invivo study using animal models, you have to determine the lethal dose (LD50)

You could decide the concentrations of your plant extract after finding out the concentrations of that same plant extract used by other researchers. Alternatively, decide a concentration and vary it so as to determine the minimum and maximum inhibitory concentration.