As for a particular range of treatments we observe a hyperbolic/bell curve. Which region do we select from the graph for experiments? Is there any particular criteria for dose concentration selection in animal cell culture experiments?
And what is the effect of your substance? Does it inhibit profilferation with increasing concentrations?
For your further planning you have to consider whether you want to have this effect or not. If not, you should choose concentrations in a range where proliferation is not affected!