"Field curvature" is a 3rd or 5th order mathematical construct, a polynomial approximation to the image profile of best focus over the image format.  It can also be approximated by tracing two real rays very close to and either side of the chief ray, two in the XZ (sagittal) plane and two in the YZ (tangential) plane, and calculate their intersection height and distance (Coddington's method).

These are both approximations to field curvature, though.  The true image surface profile is obtained by using full-pupil polychromatic real-ray or diffraction-based analysis and a defined image quality criterion based on system needs.  That criterion can be minimum real-ray RMS diameter, MTF at one or more spatial frequencies, encircled/ensquared energy at some radius, or other method.   Each of these criteria may have a slightly different curvature profile.

Your system likely has one or more image quality metrics, and you use ray-based or wave-based analysis to determine the back focal length (BFL) that meets or best approximates each metric.  If interested, the real field curvature profile can be determined by using a macro to optimize BFL for each of dozens or hundreds of field points.