C++ issue with creating a string

void RTSSHOW::RTS_Init()
{
  SERIAL_ECHOLNPGM("RTS Init");
  AxisUnitMode = 3;
  active_extruder = active_extruder_font;

  #if ENABLED(AUTO_BED_LEVELING_BILINEAR)
    bool zig = false;
    int8_t inStart, inStop, inInc, showcount;
    showcount = 0;
    // added by John Carlson
    auto mstr = (char*)""; // here is the string I'm trying to build

    //settings.load();
    for (int y = 0; y < GRID_MAX_POINTS_Y; y++)
    {
      // away from origin
      if (zig)
      {
        inStart = 0;
        inStop = GRID_MAX_POINTS_X;
        inInc = 1;
      }
      else
      {
        // towards origin
        inStart = GRID_MAX_POINTS_X - 1;
        inStop = -1;
        inInc = -1;
      }
      zig ^= true;
      for (int x = inStart; x != inStop; x += inInc)
      {
        sprintf_P((char*)mstr, PSTR("%s %f"), mstr, z_values[x][y] * 1000);
        RTS_SndData(z_values[x][y] * 1000, AUTO_BED_LEVEL_1POINT_VP + showcount * 2);
        showcount++;
      }
      sprintf_P((char*)mstr, PSTR("%s %s"), mstr, "\n");
    }

    RTS_SndData((char*)mstr, AUTO_BED_LEVEL_MESH_VP);
    queue.enqueue_now_P(PSTR("M420 S1"));
  #endif
}


// declarations for the RTS_SndData function

void RTS_SndData(void);
    void RTS_SndData(const String &, unsigned long, unsigned char = VarAddr_W);
    void RTS_SndData(const char[], unsigned long, unsigned char = VarAddr_W);
    void RTS_SndData(char, unsigned long, unsigned char = VarAddr_W);
    void RTS_SndData(unsigned char*, unsigned long, unsigned char = VarAddr_W);
    void RTS_SndData(int, unsigned long, unsigned char = VarAddr_W);
    void RTS_SndData(float, unsigned long, unsigned char = VarAddr_W);
    void RTS_SndData(unsigned int,unsigned long, unsigned char = VarAddr_W);
    void RTS_SndData(long,unsigned long, unsigned char = VarAddr_W);
    void RTS_SndData(unsigned long,unsigned long, unsigned char = VarAddr_W);