Phosphorus defiency I believe

Hi all, I have a white widow autoflower in a DWC system. This is my first time growing with dwc and indoors. It looks like I have a phosphorus defiency? I am using general hydroponics 3 part system with cal mag as well. Was wondering if anyone has a guide on how much nuterents to use on a plant of this size? The plant is about 20 days old. I flushed tonight with just ph ed water. Any info on how to fix this and proper ppm schedual would be great thanks in advance.

I have the same problem and was told its cal mag def

calcium.