How to read voltage and use the power at the same for (EX. Servo)

Yes we got that bit. The thing that is wrong is that you only have a single series resistor. That will not affect the voltage you read. You need a potential divider to measure voltage and then some calculations to turn it into a resistance.

To a great extent the accuracy will be determined by the accuracy of your fixed resistor.
I think @Koepel has the right idea here.