If you have subtracted a background value, then the bottom plateau of the curve must be 0. Prism doesn't know this unless you tell it. Make Bottom a constant equal to zero in this case. From the data table or a graph, click Analyze. Choose nonlinear regression from the list of regressions.
Prism fits the data to the equation below.
If you choose the equation with a standard slope, HillSlope is fixed to 1.0.