In case the description of the problem in my previous post was not clear enough, I will rephrase.
For a given value of battery voltage (Vbat) divided by the resistive divider, the program inside microcontroller outputs a voltage display value (Vdis).
The function Vdis(Vbat) is most probably a linear one like so:
Vdis=a*Vbat + b, where 'a' is the divider factor and 'b' is an offset.
Since right now Vdis is constantly 0.3 V lower than Vbat, meaning it has a negative offset, the correct function should be:
Vdis=a*Vbat + b + 0.3
Problem is I don't have access to firmware to modify the offset of the function. I only have access to 'a' value. Sadly that would only change the slope of the function, not the offset like I would want to. See graphs attached.