Calibrate Photoresistor

//const int Button=7;
const int PhotoPin=A0;
//this way, it behaves like analog instead of other stuff
const int LEDPin=5;
int ReadValue=0;
int MinValue=1000;
int MaxValue=0;
//these two int max and min;
unsigned long MyTime;
void setup()
{
  pinMode (PhotoPin, INPUT);
  Serial.begin(9600);
  //serial initializes it
  digitalWrite(LEDPin,HIGH);
  MyTime=millis();
  while (millis()-MyTime<=5000)
  {
    ReadValue=analogRead(PhotoPin);
    if (ReadValue<MinValue)
    {
      MinValue=ReadValue;
    }
    if (ReadValue>MaxValue)
    {
      MaxValue=ReadValue;
    }
  }
  digitalWrite(LEDPin,LOW);
  // put your setup code here, to run once:

}

void loop()
{
  ReadValue=analogRead(PhotoPin);
  ReadValue=map (ReadValue, MinValue, MaxValue,255,0);
  ReadValue=constrain(ReadValue, 0, 255);
  //constrain helps make sure it doesnt flicker anymore
  //these numbers indicate min (0) and max (170) and it inserts it into the readvalue)
  Serial.println(ReadValue);
  analogWrite (LEDPin, ReadValue);
  // put your main code here, to run repeatedly:

}