Calculate IC 50 using excel - (Nov/15/2011 )

Hi,
Somebody please help me to calculate IC 50 using excel, I have MTS assay datas. I have used conc both the doubling 1uM-800uM) dilution and 10 fold dilution(1pm to 1mM) . In each case should I plot (conc) the x-axis in log scale. Should I look at 50% inhibition of cells or 50% effect of the drug. Please explain step by step.

What I do Is I plot the log of the conc. on the X axis and the ABS in Y axis, then I select a range.. where I can see a linear function and on that range I take 3(the middle and the sides) of my dilutions to repeat the test with a postive control and a negative control ... or what I'm doing is trying with the 3 dilutions and see how the assay respond (and you choose the best of those 3 dilutions)