GraphPad Prism can do what you need. You can fit
a curve by either linear or nonlinear regression.
Enter unknown values just below your standard
curve, and Prism will interpolate the values from
the curve and give you the values. You can enter
X and it will compute Y, or enter Y and it will
Prism is available for both Mac and Windows. More
information and free demos at:
President, GraphPad Software
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