One problem with using large resistors is the time constant needed to charge the capacitor (which you will need so the AVR sees a low enough source impedance with its ADC). That can run into seconds to stabilize depending on the RC constant. So a better way is to use lower resistor values, like 10k/20k, and only turn the P-FET on when measuring. The RC constant for that is in the order of ms.
Mark.