Corn Cockle - marks the 3rd place racer.

Calla - accurately marks racers that have not been marked with a double circle or a special mark by other predictors.

Since Corn Cockle already marked the 3rd place, the rest is between Calla and Nazuna.
Naturally, Calla's double circle has the best chance of being first.
This also means that Nazuna's double circle needs to be marked just in case, since Calla didn't count that racer.