Please correct the GDL Reference Guide; GLOB_CUTPLANES_INFO returns the zzyzx minus the cutplane height, not the cutplane height itself.
Use + GLOB_CSTORY_ELEV - GLOB_HSTORY_ELEV. This will give you the height from the FPCP of your current story, rather than is being static to the objects zzyzx value.
It`s working okay, i used it to display the FPCP value of a floorplan, so its need to be a static value that is not influenced by the objects elevation (object is just there to show the value in floorplan- like a label). Adding them together made it work properly, but i will do some test with your suggestion.