hi ge90,
we operate b777-200IGW with RR engines, we use a cost index of 130. this is a fix no. (though it shouldn't be the case) which was choosen cos it comes closest to LRC at optimum.
ideally, there should be a specific cost index for each sector but apparently, it is too tedious as fuel price fluctuates. by the time they work out the cost index, it would already be out-of-date. at least thats what our company told us.
SR