Am I underfeeding?

Reddish stems, yellowish veins, dark green leaves and tips yellowing. I think it would need a bit of CalMag in my experience esp in beginning of flowering.

1500 is usually too high. You say you feed everyother watering. If you check run off from feed it may be run off from just water. Have you checked after just water? I grow in Coco and understand the difference but I may not be right about run off numbers in your case

Looking good, but I certainly would not be feeding any heavier, just keep doing what you are doing.

any deficiencies in leaves? they look okay to me. if what you provide can't keep up with growth, you know you need more. Syomptoms will arise. The upper limit is a bit more complicated. You can give more than the plant needs and gain no benefit to growth rate before you see toxicity issues. they've proven this by measuring the amount of mass of nutes found in the nutes vs what is added to media. So, you ramp up slowly.. wait a few weeks or even individual grows - observe and adjust if needed.. if no toxicities arise, keep amping upward, slowly. Take note of growth rates --this is not so helpful unless they are all clones of the same plant, however, but can give a glimmer of good info otherwise. Ratio of what you feed matters just as much as concentration you feed at... even your average VPD will impact how concentrated your feed should be. It may take some adjustment in this regard to max out fertilizer applied, too. just realize any adjustment may take 2-3weed or even months to see the full effect. A slow buildup can take forever to see - and going the other way too. Slowly progresing problems are close, but no cigar. Only a small adjustment would be needed in these contexts to fix them. The longer you have to wait to be certain you didn't create a new problem, the better in many respects. Just don't get into the mindset that shti happens overnight, because when fine-tuning fertilization it's something that plays out over months. Take notes of what you add.. how often you add.. take note of environmental factors -- light, temp+rh(VPD) etc.. you'll see correlations there. IF the plant is drinking faster in a higher VPD, it needs a lower concentration dissolved in media to match. e.g. let's assume everything is fine-tuned, but then VPD shifts upward. the plant is not takign in 1.2L per day instead of 1L. That means it's taking in 20% more mass of building blocks than normal... The rate of use of these building blocks does not increase just because the plant drinks more. So this is how a VPD change and no corresponding concentration change can result in a toxicity building up in the plant. Can't tell you how many times i've read "i've done the exact same things" ... no.. you probably have not unless you are in lab-precise envioronmental conditions. So many variables are relevant... Their behaviour may have been consistent, but that is only a small portion of the equation at play. It's not about "us." Ego and bias only fucks us up.