Do this from the Output options dialog - press Output from the nonlinear regression parameters dialog. If you calculate X from Y, beware of a possible ambiguity. It is possible that two or more points on the curve have identical Y values but different X values. In this situation, Prism will report the lowest of the X values within the range of the standard curve, and will not warn you that other answers exist. You will get more accurate results if you define the curve with more line segments.
Prism defines a curve as a large number of points.
To find unknown values, Prism linearly interpolates between the two points on either side of the unknown value. If you define the curve with more line segments, the interpolation will be more accurate. To increase the number of line segments, click Output from the nonlinear regression parameters dialog.