Let me try to explain it. There are 2 'rules':
-In any 8 members party, if you party average level is lower than the mob level you are hunting at, then when you kill PARTY mobs, you'll get double exp. Example: Say you are hunting at Ujigi, which are lvl 50, and you have 
3 party members one is 
54, the other is 
42 and the other 
50. The average level of the party will be: (
54+
42+
50)/
3 = 
48,67. So, as the average level of the party is 
48,67, then you will get double exp when killing party mobs which are at a higher lvl than 
48,67. Therefore when killing party Ujigi mobs(which are lvl 50), you'll get double exp.
-The 'best' difference of level between the member being powerleveled/farmed at your party and the monster they are being leveled/farmed at would be around, if I recall it correctly, 
10. So at Ujigi (lvl 
50) the best level for your char to be powerleveled there would be around 
40, because 
50 - 
10 = 
40.
This is what I know at least, don't know if there are other 'rules' related to this. Any question just ask. 
