Periodic evaluation is made of the two major variable costs for an aircraft's operation, i.e. Maintenance and Fuel price.
Projecting Maintenance cost is very predictable, whereas Fuel cost is a constantly varying 'wild card'.
In our company, Flight Operations make the decision. Usually, one constant CI is used globally reflecting average world-wide fuel prices. We have a few 'Sector Specific' CIs where schedule is tight with thru connections, where the commercial disadvantage of running late is balanced against operating costs. Also, if more than 15 min late, operating crew have 'carte blanche' to bump up the CI to a more 'high speed' profile.
Be wary in comparing the replies that you may get here topcat450, the 'numeric' value of CI varies greatly between aircraft, even from the same manufacturer, e.g. 40 for a B737 may be the equivalent of 90 on an A340, etc.
Regards,
Old Smokey