All A320 / A321 airlines I have flown with used a generic C.I. of 35. That was 10+ years ago before fuel prices really took a hike.
Sometimes there were multiple ways to achieve efficiency for my airlines, including high speed operation following late departure, when anticipating ground delays between sectors including slot time difficulties. Of course the preference would be to get back on schedule with greater efficiency during the turnarounds rather than burning extra gas doing it airborne. Some years ago I wrote a comprehensive post re uncommonly used methods of achieving efficient operation. Accurate calculation of C.I. for each long range flight is probably more significant than an 8 or 9 flight hour multi sector day where a bit more animal cunning can be more beneficial.
Just to repeat one old favourite. Tanker fuel to reduce refuelling time. Direct tracking and high speed descent. Inform company and crew of short turnaround regardless of company rep insisting no need due long slot delay. Push back and hold so tug unavailability will not delay ops. All checklist completed up to just before engine start. Inform company and clearance delivery that you can be airborne within 10 minutes of getting an improved slot.
Let us say that Air France ask for pushback just as their slot is about to expire and they are caught out with no tug, cargo doors still open with baggage to load etc. When their slot is cancelled, don't wait for the AF response, jump in there advising ready to taxy in 3 minutes. Worked for me close to 100% of time from Canary Islands to BRU, turning a 2+ hour turnaround into 45 minutes. Means I could leave BRU late and return early. Better than any C.I. if another aircraft is U/S or late and company sweating on getting your machine for another flight. Also very handy if duty hours need trimming to prevent extended rest period following the flight. This stuff is pure gold for a holiday charter company with tight operating margins on everything.
Direct tracking, visual approach, high speed descent and reduced APU ground probably offset additional tankering burn.
Not going for thread drift, but just an example indicating that C.I. is not the simple answer to cost saving. There are other ways to save costs. This is just one.