Help with Aref reading on Mini Pro 3.3V

Using Arduino Project Guidance
2

To read the battery voltage, I use a method similar to Nick Gammon's. Do not mess with AREF.

// Read 1.1V reference against AVcc
// return battery voltage in millivolts
// must be individually calibrated for each CPU

unsigned int readVcc(void) {

  unsigned int result;

  // set the reference to Vcc and the measurement to the internal 1.1V reference

  ADMUX = (1<<REFS0) | (1<<MUX3) | (1<<MUX2) | (1<<MUX1);
  delay(10); // Wait for Vref to settle

  ADCSRA |= (1<<ADSC); // Start conversion
  while (bit_is_set(ADCSRA,ADSC)); // wait until done
  result = ADC;

  // second time is a charm

  ADCSRA |= (1<<ADSC); // Start conversion
  while (bit_is_set(ADCSRA,ADSC)); // wait until done
  result = ADC;

  // calibrated for my Miniduino

  result = 1195700UL / (unsigned long)result; //1126400 = 1.1*1024*1000
  return result; // Vcc in millivolts
}