Your data shows the following feature:   Isolate Min / Control Growth 280 0.441905 281 0.731581 284 0.652847 i.e. the half growth is barely or not at all present in the tested range. From experience, I wouldn't expect reliable EC50 estimates from such data. Moreover EC50 estimates would be very sensitive to the choice of the growth curve function, and any estimate would be disproportionnately influenced by the growth measured at the highest concentration.   If you have other data exibiting lower Min/Max ratios, use them as test cases to develop your estimation method. Because with the data above, you are going to run into all kinds of problems. ... View more