Add a voltage reference to the Vref of MCP3008

[Geocali]

this way you eliminate the error factor added by the voltage changes of the source, of course now you will have to use 2 voltages to feed the circuit but the improvement of precision is considerable. The more expensive the ref the better the precision. (I have been using a piece of code in the atmega328p that allows to compare the vcc with an internal ref of 1.1v, it is not perfect but using some batteries that were practically new. I have measured an error of + -100mah which is acceptable to me from moment)

https://datasheet.lcsc.com/szlcsc/Linear-Tech-LT1029ACZ-PBF_C117263.pdf