Depending on what voltages you are measuring and whether you care about input impedance , you can get by happily with just a voltage divider .
For accuracy use the processor internal reference , with that code the accuracy depends on the stability of the 5v supply to the Arduino , and your resistor choice doesn’t make use of the full range of the A/D.( 15v , gives around 1.2v at the input to the A/D , the resolution will therefore be poor )
As you are using 10k/100k , then you are better off just using a voltage divider on the analog input .
As above , draw out the circuit and give the outputs , indication of voltages etc