You might want to check run off water for the real PH, maybe I'm seeing things.
It might be the strain or something, but those leaves look like they are suffering from a PH issue.
So with the PPM at 300 that is fine, I do it like this.
Week 1: Tap water (30-75 PPM)
Week 2: Root nutrients (50-125 PPM)
Week 3: Plant food (150-250 PPM)
Week 4: Anything left (250-500PPM)
Week 5: No burned tips? Up PPM by 100)
Week 6: No burned tips? Up PPM by another 50-100)
Normally people switch to flower by now, and you just start over at week 2 PPM levels for flowering nutrients (Like Rhino Skin, Bloom Booster, Big Bud, anything for flower)
and start working your way up every week.
The max PPM you want, is when your plant has tip burn. Normally that is 700-800PPM (Depending on nutrients used) never go above 1000PPM unless you are a master grower.