Newbie grow basics please assist!

Shazzabee Okay rightio then. What a bleddy day to be alive Plants have been re-potted from their initial little L9s which they were directly germinated in (with 100% coco) to now include a 70/30 coco/perlite mix. They are drying back better now and not as compacted which is grand. There was loads of roots when I repotted. I have a truncheon to measure ppm Tap water is 100/150ppm I have been feeding 150/200ppm and now upped it to 350ppm after a question on increasing ppm EC on the truncheon shows 0.6 and when I catch the run off and measure it, it is 0.4 PH of water after nutes is 6.0 I only have the 0.5 chemical bottle colour scales and not a digital meter Nutes used are Athena grow A+B and as of today I now have CaMg to introduce as I'm aware of the worsening brown situ. I started getting brown specs and such on first leaves so advice was to flush with PH water only. Some loved it others are still in a decline - pictured Some plants look miserable others look happy enough albeit pale, I don't know whether they are pale from stress or lighting. Purple stems - some people saying not enough light - is this a Magnesium deficiency? Or is it simply due to running LEDs? Lighting - I have two LEDs set up at 250W each. Fed from independent ballasts (2x 600W) They are the Adjusta-Watt Retro 720W I previous had just the one but when the pots upsized I needed another light in there Lux Meter - saw a post on Migrolight so I used a Lux Meter to measure and it seems mighty high without the rough 0.02 multiplier to get it to PAR figures. Humidity sitting between 60%-70% Could anyone help with the following please - clarification on the purple stems - how to dose the CaMg. Will adding it in alongside the A+B be adequate or do I need to take more robust measures? - Any advice about the lighting situation please 🙏 - what can I do about the shrivelled up looking plant? Do I need to PH and CaMg it only? Any other feedback/guidance/prayers are appreciated