Level difference between group members

What is the maximum difference between characters in a group that can still get exp? I thought it was one half of the level of the highest level person but that does not seem to be correct. I have a 30 BL and I am trying to level up a cleric to duo, so I am curious what the lowest level could be and still get exp for both. Thanks for the assistance!

 

1.5x ie: 20 with 30

 

So it's kind of the opposite. It's one half of the Lowest level. Example:

20 can group with 30;
16/17 can group with 35;
...
40 can group with 60

 

Alright, got it bass ackwards. Thank you folks, have a bit more grinding to do!

 

Looks like ogre math to me...

 

Ogre math stops at 20 (or less if they are missing any fingers or toes).

 

Like Bum said:
Its a fixed maximum scale factor or ratio.
[3/2 or 1.5 (level)] going up
or the reciprocal
[2/3 or 0.67(level)] going down