potVal = map(potVal, 0, 1023, 0.0, 5.0);
potVal = map(potVal, 0, 1023, 0, 11);
PS. When you are choosing a replacement for digital pin 1, you might want to avoid digital pin 13. That pin already has an led attached to it, on the Mega board. If you drive two LEDs with one pin, you could overload and damage the pin.
That's funny! I thought that the Mega 2560 - like the UNO (and even most fake UNOs) had the LED on pin 13 buffered by the op-amp.