How to load scal factor external to the code?
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.