Question: How much of the hand/fingers should be washed for mayim acharonim?
Answer:
The Shulchan Aruch (Orach Chayim Siman 181 Se’if 1) writes that there is an obligation to wash mayim acharonim [at the end of a bread meal, prior to birkas hamozon]. One can use any liquid for mayim acharonim (ibid Se’if 9) – if one does not have water available with which to wash (Pri Megodim EA s.k. 7). When washing mayim acharonim one should wash the fingers until the knuckles. The Shulchan Aruch (ibid Se’if 4) requires washing until the second joint of the fingers, however, the Kaf Hachayim (Sofer, Siman 181 s.k.17), based on the Arizal writes that all three joints of the fingers should be washed. Both the Kaf Hachayim (Palagi) and the Ben Ish Chai write that the four fingers should be separated from the thumb while washing. Unlike netilas yodayim prior to the meal, there is no minimum requirement of water that needs to be used for mayim acharonim, provided that the water is an adequeate amount to wash with.