First time DWC, day 70, 3 weeks in flower. Nasty rusty spots appear on lower leaves. Is this CalMag or else? Please help understand and fix, thank you :)

Yes, that could be Caclium deficieny (not Mg). Because you are soilless/hydro there's an easy way to figure this out. Calculate the ppms of Ca you provide from all your products added together. If it's greater than 100ppm, it's more likely something is locking it out than a physical deficiency. P is the most likely candidate to lockout Ca. Probably need well over 100+ ppm of P to inhibit Ca, but that's just a guess. ("mulder's chart" is reference) there are apps that can calculate and tabulate your elemental ppm by simply typing in gauranteed analysis info and dose of each product. Growbuddy? hydrobuddy? something akin to that moniker. They also have an online calculator, if i still have it in my bookmarks i'll copy it below. https://www.angelfire.com/cantina/fourtwenty/articles/profiles.htm -- not the one i was thinking of but could work. the better ones will take info from all your product and add it all up. this one looks like you'd ahve to do each product seperately then add together. important note to consider: "When no weight information is available for liquid fertilizers the nutrient profile can be understated by as much as 25%." Do not compare these ppm to a TDS reading. TDS readings are not accurate ppm for this context. Dependin on manufacturer and nothign else, it may use significantly different conversion factors because the ppm reading you see is just EC multiplied by one of 4 or 5 different factors... a bit whimsical in nature, lol. TDS ppm readings are mostly useless except to see if consistency is maintained. The reading is consistent, it's just not accurate. That website also mentions this tidbit. What i type below is a very loose set of values that may vary greatly.. or if another ratio of nutes works out well could result in different ranges that are ideal... N 120-130 (100 or less in flower) P 40-60 K 180 ish Ca 100+ mg 75ish S 100+ If anything is wildly different, in this case wildly higher while Ca is somewhere nearby, it is more likely lockout. Drastically different ratios may result in different "safe" ranges for these PPMs. Again, mulder's chart is the reference as to why that is the case. the more similar to these ratios you feed, the more useful these numbers become. All imean is there are more than 1 way to skin this cat. I wouldn't assume to know which is the best, but this formula works for nearly all plants i've grown with few requring anything but minor adjustments.

hum... 2nd answer that a random user is name dropping fertilizer brand METROP, Any brand can be overfed and cause damage to a plant.

Dark leaves aren't always a sign of overfeeding, some strains just have darker leaves. Nute burn will display more on your tips, not random spots like that. Also 1.6 EC isn't high at all. I use TA(well General Hydroponics but same diff) and your ratios look good. I'm not familiar with the Plagron additives you're using, if it's nute related I'd look there. I see a lot of topoffs mentioned in your diary but no mentions of tank changes anytime recently....how often do you? Might consider doing it at least every two weeks if you're not already. I can get a little lazy about tank changes myself but it's never a bad idea if you got issues popping up.

Use Metrop nutrients you will never have an issue.

Looks to me like you're running a little bit hot. Dial back on your nutrient levels. Adding to what was stated below, new growth will tell you if you solved your issues. It will take some days to notice any changes.

Beginning of burning from over feeding, not a cal/mag issue. Anything you change will take 10-14 days to show in the leaves, it will not happen overnight! Leaves are waaaaay too green...........bordering on nitrogen toxicity.