First,do MTT assay or trypan blue assay on MCF7 cells by using tamoxifen compound. then determine the IC50 value then use that value for next level of your experiment.
Initially,we don't know , what is the ic50 value of our compound, so prepare from higher concentration to low concentration.treated the cells with 100 micro molar to 10 micro molar.if there is no viable cells in 10 micromolar means,treat the cells with >10 micromolar concentration.
take the OD value of the MTT experiment then calculate ic50 values.