void setup()
{ 
  Serial.begin(9600);
}

void loop()
{  
  int sensorValue;

  sensorValue = random(0,1024);     // analogRead(A0);  // 0...1023

  Serial.print( "Bad value: ");
  auto n = Serial.print( sensorValue * 5000 / 1023.0);  // <-- Bad
  Serial.print( " mV.");

  for( int i=n; i<8; i++)
    Serial.print( " ");
  Serial.print( "Good value: ");
  float voltage = float( sensorValue) * 5.0 / 1024.0;   // <-- Good
  Serial.print( voltage);
  Serial.println( " V.");

  delay(1000);
}