There are several other aggro rules that you can tweak. There's a melee range mod that matches your description above and several others:
Code:
RULE_BOOL ( Aggro, SmartAggroList, true )
RULE_INT ( Aggro, SittingAggroMod, 35 ) //35%
RULE_INT ( Aggro, MeleeRangeAggroMod, 10 ) //10%
RULE_INT ( Aggro, CurrentTargetAggroMod, 0 ) //0% --will prefer our current target to any other; makes it harder for our npcs to switch targets.
RULE_INT ( Aggro, CriticallyWoundedAggroMod, 100 ) //100%
RULE_INT ( Aggro, SlowAggroMod, 450 )
RULE_INT ( Aggro, IncapacitateAggroMod, 500 ) //mez, blind, charm etc etc
RULE_INT ( Aggro, MovementImpairAggroMod, 175 )
RULE_INT ( Aggro, SpellAggroMod, 100 )
RULE_INT ( Aggro, SongAggroMod, 33 )
RULE_INT ( Aggro, PetSpellAggroMod, 10 )
RULE_INT ( Aggro, StunAggroMod, 750 )