Anybody know the leeching formula for EXP at LHC/LKC or SH?

Unfortunately I can't recall the sizes of the parts of the leech formula, so what I just told you is pretty useless. Hopefully this bump will get the attention of someone who remembers, though.

If I remember right it's 80-20, meaning 80% of the exp is split fairly between the party members (based on level) and 20% is split based on damage.

In a 2 person party this means a pure leecher gets half of the 80% and none of the 20%, so 40% of the exp.
In a 4 person party, leechers each get 20% of the exp.

Level difference goes like "add all the levels, then party member gets a percent equal to their contribution." So level 100+200 means the level 100 gets 33% and the level 200 gets 66% of the exp.