True.
But i guess it is all by experience on individual tank and personally believes.
I play around with the amount of biopellet, its flow rate, use of GFO, carbon and etc, which results in different behaviours of animals and algae in the DT. From there, it sort of gives me a clue on what is the best for my corals.
As such i start not to worry about testing PO4 and No3...