based on my experience at first you should select at least 6 or 7 concentration of your toxin (for example 0, 0.5, 1, 1.5, 2, 2.5, 3 %) and do an experimental test on your target organism. after that you should count number of mortality per each concentration and for finding LC50 you should enter data of mortality, concentration of toxin and total number of target organism in data sheet of SPSS and use analysis of probit regression. Software will calculate LC50 for you.