little oops

Marlin/Marlin_main.cpp

@@ -5152,7 +5152,7 @@ inline void gcode_G28() {
         S2 += sq(z_at_pt[0]);
         N++;
         if (!pp_equals_1) // std dev from zero plane
-          for (uint8_t axis = 1; axis < 13; axis += (pp_equals_2 ? 4 : 2)) {
+          for (uint8_t axis = (probe_mode == -2 ? 3 : 1); axis < 13; axis += (pp_equals_2 ? 4 : 2)) {
             S1 += z_at_pt[axis];
             S2 += sq(z_at_pt[axis]);
             N++;