I need to find x-values according to a four parameter logistic equation ( y =
(a-d)/[1+(x/c)^b]+d

How can I do this? I am not very familiar with solver. I have used XLSTAT
to find my four parameter curve and my a, b, c, and d values.

To be a little more descriptive (as I am really hoping to get some help here):

I am working with ELISA data. I have a calibration curve, therefore I have
concentrations(x, actually in LOG) and their response (y). I will generally
get a response (y) from my assay and want to find my estimated concentration
according to the calibration curve. (Like using the forecast function, but
it is not linear).

Can anyone help me with this?

Thank you,
Laura