Output Impedance of a Digital Output
The output impedance varies a bit with current, but only slightly, so you can use the value given in the datasheet for higher currents too. (Note that the 160 mV is a typical value, and that the value may be as high as 400 mV. Then the impedance will be 25 &Omega;, so it's not precise anyway.)  



This graph is from the ATmega64 datasheet, and shows output voltage versus sourced current. If the output resistance would be constant we would have straight lines. They're slightly curved, but the difference in resistance is far less than the tolerance given in the datasheet.