tinman13kup:
The ground path for your divider (as shown) is through the mosfet. With the mosfet OFF, you have no ground, and all points of the circuit will reach max potential--12VDC. Try and see. It's the easiest way to learn.
I have tried it, the largest voltage measured under any circumstances @ middle of divider is 5.2V....if it got 12V I would have fried the pin.
I am trying to gather parts for allan's circuit now, not sure I have all the comps....
and thanks allan for the description of the circuit, that's helpful.