As I get paid by the minute... I wish we used a CI of 1.
However our range is between 11 to 15 depending on sector length.
Average ECON speed for the NG varies from .77 to .78 at the higher altitudes, or about 5 knots... At the end of the day, not very significant for a 4 hour flight.
There are times during ETOPs operations, we are requested to fly a given and RTA request which throws the CI out the window all together... even though not resulting in much a total fuel burn deviation from the plan.
Bigger aircraft... larger difference however.