How do I change the Battery Percentage that corresponds to a Voltage in Android
Let's look at the challenges you would face in achieving correct reading, rooting is not a magic key that let's you do everything easily:


The battery capacity (mAh) is coded in two places - the ROM and kernel (see Where are the battery capacity files located?) Only OEM would know from where the values are picked up, so you would need to have a custom ROM and kernel that stores this higher value. You would need to do this own and this is definitely not a trivial effort. ROM "chefs" wouldn't be interested in this (not appealing or in demand). You say you aren't a software person, so it's safe to discard this path.
The charging algorithm is also proprietary and not open source given the competition ( I can  charge from xx to yy% in so many minutes is the sales pitch). So that's your next huge challenge since the charger, charging cable and the device need to be in sync with the algorithm. Impossible for you to replicate. See the charging algorithm 
(Source
) of an Anker power bank to get a sense of how challenging it would be to figure out on your own. 
Charging incorrectly is going to damage your new battery and the purpose of acquiring it is partly defeated. This would be the result since you aren't easily going to achieve the first two points. 


It is for these and probably more reasons that third party sellers who provide extended batteries do not attempt to show the correct percentage. Read the reviews of any such product and you will find complaints around this

What you can do


Use a battery bank of desired capacity. 
Experiment at various charging speeds and arrive at a rough correlation between shown and actual voltages. 
Rig up hardware based system as you suggested in your question (off-topic on this site)