It should be noted that Rbottom must include the Z_In of the voltage source.
Assuming a battery, this would be the battery internal resistance, Rs is the Z (impedance) which is very low compared to Rbottom and can be typically excluded.
With an AC input, Z is complex and a capacitor is generally used to bypass the higher frequencies, that is, low-pass filter.
Finally, the two resistors are generally higher values to prevent "loading" the V_in, but too high (> 20K Ohm each) may not match the Atmel suggested 1OK Ohm analog input resistance.
Good news is that for most common input sources, one can use the general two resistor equations.
Ray