UBL Marlin settings to cover whole bed
PROBING_MARGIN and MESH_INSET make the effective probing area smaller, so if you want to have more area, you should reduce the value of these constants.
As of Marlin 2.x, the probing area isn't defined directly by the firmware configuration settings, but calculated, based on the probe offset settings. The constants you mention are reducing the probing area to keep the carriage/nozzle on the build surface.
If you have enough space on your printer to accommodate probing the whole bed, you could minimize the marging and define edges:
#if PROBE_SELECTED &amp;&amp; !IS_KINEMATIC
  #define PROBING_MARGIN_LEFT PROBING_MARGIN
  #define PROBING_MARGIN_RIGHT PROBING_MARGIN
  #define PROBING_MARGIN_FRONT PROBING_MARGIN
  #define PROBING_MARGIN_BACK PROBING_MARGIN
#endif