Is this the start of a cal/mag deficiency?

Ok let’s just clear up - there’s no such thing as calmag deficiency. There is calcium deficiency, there is magnesium deficiency. There is both calcium and magnesium deficiency as well but it’s important to separate the two from each other for proper diagnosis.. often times it’s one or the other..

I looked at your diary on this grow to see what the cause might be - but your pH is fine and you don't seem to be giving TOO much nitrogen (although if I were you, I'd be dropping the amount of Bio Vega)... so the addition of some calmag might be good... Just don't look for a change in THESE leaves - they won't get better - but you shouldn't see the symptoms show up in new leaves after about a week.. Good luck! Looking really good!

rusty spots are more ph issue first sign, which can lead then to calcium issues. by picture, you are growing under led, i noticed that you have to use small amounts weekly, like 0.5ml/l once a week, does do good prevention. other leaves are very healthy and not overfed, so its not nitrogen toxicity caused issues. i would say you are safe if added a bit extra cal mag and make sure your ph is right and meter was calibrated.