You seem to be making a meal out of reading an analog pin voltage - why not simply:
#define Vsupply 3.3 // or whatever it needs to be
#define R0 10000.0 // name your constants at top level, don't sprinkle throughout the code
#define RESISTOR_PIN A0
float read_resistor ()
{
float Vpin = analogRead(RESISTOR_PIN) * Vsupply / 1024 ; // one line, simple to read
return R0 * (Vsupply - Vpin) / Vpin ; // simply clear calculation using voltage ratio
}
(Which would work if no other resistors on the pin)