brackets are what you want sir, good ole brackets.
a1 setPos [(getPos leader heroes select 0) +/- x_offset , (getPos leader heroes select 1) +/- y_offset, 0]
just to explain a wee bit, the x_offset is positive if it's to the right of your target unit, negative if it's to the left.
similarly, y_offset is positive when it's in front, negative when it's behind.
so for 500 meters behind the leader of the group heroes -
a1 setPos [getPos leader heroes select 0 , (getPos leader heroes select 1) -500, 0]
EDIT - note, i'm not quite sure if that's left/right according to east/west of the map, or according to which direction the target loon is facing....