SEN0169 very inaccurate on calibration

//input voltage tested at 4.96v, temp 23.9
//lowDrift = 2.78, highDrift = 0.73

static float low_pH_buffer = 4.0;
static float lowV = 0.35;

static float high_pH_buffer = 7.0;
static float highV = 1.79;
float m, c;

void setup(void)
{
  ...

  //create a two point offset in the format y = mx + c
  m = (low_pH_buffer - high_pH_buffer) / (lowV - highV);
  c = low_pH_buffer - m * lowV;
}

void loop(void)
{
  ...
  if(millis()-samplingTime > samplingInterval)
  {
      ...
      //pHValue = 3.5*voltage+Offset;
      pHValue = m * voltage + c; //plot a line in the function y=mx+c using the two voltage/ buffer points
      ...
  }
}