Try to understand the cost index and how it works is like trying to understand your girlfriend or wife all the time and it's near impossible simply because of all the managerial factors involved in deriving that special number.
In my airline we operate at CI 40 in normal conditions and 120 in case we need to speed things up a little bit; it increases your mach number by approx .2 - .3
If you really want to get into CI here's a link to an explanation put out by Airbus if you're really interested in details...
SmartCockpit - Airline training guides, Aviation, Operations, Safety