Week 5 yellowing: N deficiency or natural fade?

Running a soil grow, week 5 of flower, and the lower and middle fan leaves are starting to yellow out. Not super rapid, maybe 3-4 leaves every couple days working upward. The plant otherwise looks healthy, pistils are stacking, upper canopy is dark green. Just trying to figure out if I should be reaching for my nitrogen source or leaving it alone.

My current feed is sitting around 150 ppm N, 180 ppm P, 220 ppm K at the root zone, pH locked at 6.3. I bumped potassium up at week 4 like usual, and dropped nitrogen a bit intentionally. Now i'm second-guessing that call.

The thing I keep going back and forth on: week 5 is right around when natural senescence starts on some strains, especially heavier indica-leaning ones. But it can also be the plant telling you the N drop was too aggressive. Hard to tell from the rate alone. Anyone who mixes their own salts and tracks this closely, what are you using as your real indicator to tell the difference? Interveinal chlorosis, speed of spread, upper canopy color? Curious what people actually watch for.