Need help setting probe points for UBL (no replies)

I have a 300 x 300 bed, my probe has offsets of

X -29
Y -51

I need around 10mm clearance around the edges of the bed to avoid my binder clips. Due to the position of the probe, I can reach the front and left edges of the bed but not the rear and right edges.

What values do I need to set for the following and how do I calculate them?

  #define UBL_MESH_INSET 30          // Mesh inset margin on print area
  #define GRID_MAX_POINTS_X 15      // Don't use more than 15 points per axis, implementation limited.
  #define GRID_MAX_POINTS_Y GRID_MAX_POINTS_X
  
  #define UBL_PROBE_PT_1_X 50       // Probing points for 3-Point leveling of the mesh
  #define UBL_PROBE_PT_1_Y 220
  #define UBL_PROBE_PT_2_X 50
  #define UBL_PROBE_PT_2_Y 90
  #define UBL_PROBE_PT_3_X 240
  #define UBL_PROBE_PT_3_Y 90