precise adc values from the vref pin.

getting precise adc readings, originally from http://hacking.majenko.co.uk/node/57

long readVcc() {
 long result;
 // Read 1.1V reference against AVcc ADMUX = _BV(REFS0) | _BV(MUX3) | _BV(MUX2) | _BV(MUX1); delay(2);
 // Wait for Vref to settle ADCSRA |= _BV(ADSC);
 // Convert while (bit_is_set(ADCSRA,ADSC));
 result = ADCL;
 result |= ADCH<<8;
 result = 1125300L / result;
 // Back-calculate AVcc in mV return result;
 }

 void setup() {
 Serial.begin(9600);
 }
 void loop() {
 Serial.println( readVcc(), DEC );
 delay(1000);
 }

So now, using that, your ADC code could now look like this:
unsigned int ADCValue;
 double Voltage;
 double Vcc;
 Vcc = readVcc()/1000.0;
 ADCValue = analogRead(0);
 Voltage = (ADCValue / 1023.0) * Vcc;