How to load scal factor external to the code? We know that, we need calibration factors (scale factors) to read current sensor data (ACS758) to arduino. I am also using voltage reader using voltage divider circuit for my project. I need to build 5 such boards and need to change calibration factor in main program everytime due to tolerance of componenets. Currently for one of my board reference_factor is 0.81 and this will change due to resistor tolerance for other boards.

Because of this issue I need to maintain 5 different codes for 5 different boards. These sensor values controls on and off of certain relays. (Not shown in the code). 

can I load this referrence_factor externally through text file or as any other medium so that I need not change the code everytime.? I am using arduino Nano. 

int reference_Factor = 0.81;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
}

void loop() {
  // read the raw data coming in on analog pin 0:
  int lightLevel = analogRead(A0);
  // Convert the raw data value (0 - 1023) to voltage (0.0V - 5.0V):
  float voltage = lightLevel * reference_factor;
  // write the voltage value to the serial monitor:
  Serial.println(voltage);
}
As written in the comments you coulduse eepron, but in your scenario it has one big disadvantage: Everything has to be written at compile time (aka when you programm the Nano, you have also to fill the eeprom with the relevant values), so e.g. if you have a temperature depending factor you want to change while on the field...
I would go for a SD-card reader:


Cheap to get (2$)
Easy to handle in code (read from a file is a standard function)
using a PC compatible file systen FAT enables reading/writing onto the SD card on a PC/other device without recompiling your code.
If you switch sensors for whatever reason, just write a new scale.txt file to the SD-card, insert and reboot the nano to get the new values
you could still use eeprom as a fallback scenario


As a tip (also mentioned in the comments by Delta_G) int for something like 0.01 is not what you want - define as float and watch out for the "conversion trap" when using with other variables not defined as float.