Question about FCRV

[tonyleblanc]

When specifying FCRV, is there any standard as to the plane against which the curve is measured?  For example, is the curve the curve of the entire frame, when sitting flat on a surface, which would make the plane of reference relative to the center of the bridge (assuming the frame curves "up" from that point)?  Or, is the curve relative to some point on the shape of each eye, perhaps the GC of the box?  (Because FCRV is chiral, that would seem to suggest the curve is for each eye independently).