To calculate the resistance of the PTC you don't need to know the input voltage, as all you need to know is the ratio between the two resistors, and that's what the ADC reading really gives you.
It's therefore a good idea to use a low tolerance R2, 1% or better (a 0.5% or 0.25% tolerance doesn't cost much extra than a standard 5% one), or measure the resistor accurately and use that value.