Week 5 yellowing, N def or natural fade?

Running a Zkittlez auto, coco/perlite, mixing my own base salts at roughly 185 ppm N going into week 5 of flower. Bottom and lower-mid fan leaves are going yellow and dropping. pH is locked at 5.9, EC around 2.1. No obvious spots, no interveinal chlorosis, just a clean yellow-to-drop progression moving upward slowly.

My instinct says this is the plant mobilizing N out of older tissue as it pushes into late flower, which is textbook. But week 5 feels a little early for me to be comfortable calling it a natural fade. I've seen true N deficiency move faster and hit newer growth eventually, this hasn't done that yet.

For those with more runs logged, what's your read? Is clean upward-progressing yellow in week 5 more likely the plant doing its thing, or would you bump N a little and see if it stabilizes? Thinking about pushing my calcium nitrate fraction up slightly to add maybe 20-25 ppm N without throwing off the rest of the profile.