How to measure 6S LIPO battery juice in percentage from Raspberry Pi?
This is the discharge curve for a Panasonic NCR18650B which I believe is a li-Colbalt.  The Li-Po is a lithium colbalt.

Discharge is very linear especially with a lower discharge rate.

Li-Po Cutoff Voltage is 3.0V

So you have a 1.2V hysteresis from 4.2 full charge to 3.0 cutoff. 

Read it with a ADC board.  There are a few sub ten dollar ADC boards for the PI like this:  16 Bit ADC $9.99