Ppm talk and proper nutrient ratio

so I had some problems with a few plants I came here for help and got wonderful tips from many people and my plants seem to have come back and are looking strong again....
My question now is what ppm should I be feeding at. I was told by someone that my nutrients are very weak... indeed they are I did a full dose (5ml) per gallon of the recommended nutrients from my feeding chart and my ppm meter came out just around 400 ppm...
Grow specs
Led 1200w (306actual Watts)
1.5 gal pot
Ffof/coco mix
Roots organics master pack liquid nutrient line
Ppm-300-400
Ph6.5

If the initial problem was overwatering id guess light doses till they start taking off but you will have to read the plant and less than healthy ones complicate too many issues.

The soil part should feed for a few weeks so anything i say is more a guess and youll have to figure its schedule some.

A full or half strength dose is my goto amount, they show results of the fert quick and if not somthing else is bothering them

If your plants..."have come back and are looking strong again...." Why mess with what you're doing? If you feel you need to add more feed, up their feed some. Do it in small increments and instead of fixating on a ppm number, fixate on your plants. They'll tell you everything that number will and more. I haven't seen too many people happy with the hybrid mix of coco and soil, so good luck there. At this point, based on your pics, I would be sure you're not over watering and you should begin to address what looks like the onset of being short on Mg. Remember coco holds onto Mg and Ca...depending on how much you have in your mix, you'll need to mitigate this fact. Good luck.

1 your light is shit. No way 300w equals 1200w that's led marketing lies. I know you can do nothing about that now. Next time you buy a light keep that in mind. The stronger the light the more photosynthesis, the more nutes they need.
2 ppm and ec mean nothing with organic nutes like roots organic. Also do not try to adjust ph of nutrient solution with organic nutes. They will self buffer in the soil