Does this look like CalMag deficiency

4 week old autoflower, just in the stretching phase. Growing in a 15L medium with soil, compost and perlite. The soil should be very rich in nutrients which is why I have only fed it once at week 3 at a small dosage. I water it when the pot feels light and is dry. I keep the light scheduele at 20/4. light is mars hydro ts600 and the ppfd is showing 800 from the canopy. I am noticing some yellowing and fading on the leafs. Seems to only affect the upper leaves. This to me looks like «CalMag» deficiency, anyone else agrees to this?

It's not Ca, it could be Mg, or even iron or sulphur (unlikely). Do a foliar spray of epsom salts and if it's Mg it will turn around quickly. Since it's affecting the new/upper growth it's a non-mobile nutrient.

No such thing as a calmag deficiency.
It's likely because it's stretching , nutrients aren't keeping up with growth rate.
It should catch up with itself