Grouping levelsFollow

I'm interested in everyone's understanding of the mechanics that govern experience in groups.

I thought once you got past 28 the rule of thumb was 75% e.g. to get experience with a 50 you have to be at least 38, and 45 to group with a 60.

However someone in my guild tonight insisted it's a fixed 15 levels, and therefore to group with a 65 you must be at least 50 (not 49 as it would be for 75%).

Anyone have any experience or thoughts on this?

its 75% not 15 levels, cause I know for a fact that a 28 can't group with a 43.