I've successfully digitized graphs using Engauge Digitizer, an open source digitizer available for download on sourceforge. I will then curve fit using either Matlab or GNU Octave. I wouldn't use Excel curve fitting for anything but the simple linear and quadratic equations. You must be extremely careful to check the registration, as scanned of prints (like you would get from an old AFM downloaded from MyBoeiingfleet that has been scanned) usually have distortion.
Building a SCAP module is involved and requires access to an AFM or other primary source.