If you're not partied with someone and kill something together, the EXP is split by damage.
If you're partied with someone and kill something with someone else not in your party, presumably the EXP is split between the party and the other person first (by damage), then the remainder is shared amongst the party the usual way.
In order to leech from a party member's kill you must
Be no more than 5 levels under the monster OR
Have done at least 1 HP in damage to the monster OR
Within 5 levels of any other party member that satisfies one of the first two conditions.
Hence why you get your ~20% when you kill misted monsters. You get a normal split of EXP as if you were within leech level.
Assuming Stereo's split by level thing has been tested (it sure seems to fit, just imagining it), I'll put it up.
If you're partied with someone and kill something with someone else not in your party, presumably the EXP is split between the party and the other person first (by damage), then the remainder is shared amongst the party the usual way.
In order to leech from a party member's kill you must
Be no more than 5 levels under the monster OR
Have done at least 1 HP in damage to the monster OR
Within 5 levels of any other party member that satisfies one of the first two conditions.
Hence why you get your ~20% when you kill misted monsters. You get a normal split of EXP as if you were within leech level.
Assuming Stereo's split by level thing has been tested (it sure seems to fit, just imagining it), I'll put it up.