Week 5 yellowing, N deficiency or natural fade?

Running a soil grow under 600w HPS, week 5 of flower. Lowers are going yellow and dropping, but i'm also seeing some fade starting on the mid-canopy fan leaves. pH is holding at 6.4, runoff EC is reasonable, and I haven't changed my feed schedule. Base mix is calcium nitrate, monopotassium phosphate, and magnesium sulfate, boosted slightly for flower.

My read is that some lower leaf drop at week 5 is pretty normal. The plant is pulling mobile nutrients from the oldest leaves to support bud development, so a bit of senescence makes sense. But i'm second-guessing myself because the yellowing seems to be moving faster than I expected, and a couple of the mid leaves look a little pale rather than that clean yellow-to-drop fade you see in a healthy finish.

What's the general rule of thumb you guys use to tell the difference? I know N deficiency usually shows as uniform pale yellowing starting at the bottom, while natural fade tends to be more patchy and progresses slower. But at week 5 those two things can look really similar. Curious if anyone bumps N slightly mid-flower when this shows up, or if that's the wrong call and you just let it ride.