If I have 12 V going to a resistive divider series of (1.5MOhm) and parallel to ground (1.0MOhm), and I take that voltage divided output and route it to a pin...how much current is being driven to that pin?
Not exactly, your approach would calculate total current that flows from the power supply, your aim is to calculate that part that goes to the pin, so:
What if the output is of lets say a IC, for instance the data outputs of a ADC? The data bit output can either be 3.3 V or 0V, with a output resistance X ohms. Would that be a ideal voltage source at 3.3 with a series resistance of X ohms?