Week 5 DWC update, cal-mag issue creeping in

Running 4 Zkittlez plants in a recirculating DWC setup, 4x4 tent, 600w HPS with a 200w Samsung QB supplementing the canopy edges. Reservoir is sitting at 68F, pH locked at 5.9, and i bumped EC up to 2.1 (roughly 1050 ppm on the 500 scale) going into week 5 of flower. Plants look stacked so far, internodal spacing is tight from the LED side fill.

Problem showed up around day 32. Interveinal chlorosis on the mid-canopy fan leaves, newer growth showing some cupping too. Classic cal-mag deficiency signs, even though i was dosing CalMag at 3ml per gallon. My tap water here in Denver runs around 35-40 ppm baseline, so there's basically no calcium buffering coming in naturally.

I bumped CalMag to 5ml per gallon and flushed the reservoir with a fresh mix at EC 1.95 just to reset things a bit. Within 4 days the new growth came in clean, but a few of the affected mid leaves are still showing it. Not spreading though, so i think i caught it before it compounded.

One thing i'm wondering about is whether the HPS heat is pushing my VPD higher than i think and accelerating the issue. Canopy temp is 79F lights on, RH around 52%, which puts me at roughly 1.4 kPa. That's on the high end for week 5. Could transpiration demand be outpacing calcium uptake even with adequate solution levels?

If anyone else has run DWC in a heat-stressed environment and dealt with persistent cal deficiency mid-flower, i'd appreciate knowing how you dialed it in. Specifically curious whether dropping EC slightly to improve water uptake made a difference, or if you just kept pushing the cal-mag dose.