That's why I recommended to use a 120 Ohm resistor
and switching to the stable 2.56volt internal reference of the Mega.
Leo..
void setup() {
analogReference(INTERNAL2V56; // switch Aref to 2.56volt
}
Edit: calibration is always needed, because the resistor might not be exactly 120 Ohm and 2.56V Aref is not factory calibrated.
Please post the code that you're currently using.