You can use any public odds that you like to calculate the race's ci.

To reiterate, a maximum entropy/minimum ci race is one where all horses have identical parimutuel probabilities. A minimum entropy/maximum ci race is one which has a uniform distribution of parimutuel probabilities over the interval 0 to 1.

Ceteris paribus, a longer field will yield a lower ci. Conversely, a shorter field will yield a higher ci.

The ci can used to determine the state of the tote board as well as a metric to index the races after the race has been run. It is a metric measuring competitiveness as determined by the bettors. It is also a concentration or dispersion of the odds in a horse race. The ci does not measure the strength of field. It measures the public's perception of the competitiveness of the field.

It is a way to measure more than just the favorite to determine the public's perception of the race's competitiveness.