I'm growing the same strain at 8 weeks now. I had a cal mag problem due to long periods between waterings (4-5 days) right at 2-3 weeks. Add a tsp of cal mag (5 mls) per gallon along with shorter watering periods mine cleared up in a couple days. My deficiency wasn't as far along as yours though. The leaves won't get better it will be clear on new growth.
It's my first grow and I was able to figure this out by reading ALL the tutorials concerning this particular issue AND browsing/reading relative topics in the forum.
It's all here, you just have to read it.
These folks know what they are doing. Read and ask good questions including relative info about your grow and you will get more knowledge than you know what to do with!!!
Also check Nebulas nutrient schedule for GH trio in coco for autoflowers, it will get you to the finish line. Us new growers need a template to start with, then as we gain experience we can tweak it.
Nebula knows coco, that's her lane, as the kids say!!
Happy farming.
PS: I don't know where you are but I use 1 gallon jugs of mineral spring water from Walmart, store brand, about 60 cents per gallon. pH's at 6.0 plain everytime and it has done well by me.