The total phenolic content measurement is based on the reaction of Mo(IV) reduction by phenolics. Basically, this is the same approach as used to determine the antioxidant activity. Therefore, it should be a simple relation between these two numbers

In most of the cases where phenolics are playing important role. Among phenolics the relationship with flavonoids may or may not be linear due to availability and extraction

This is related to the composition of your extract flavonoids, tannins or phenolic acid

With correlation you can know who's responsible of the antioxidant activity