I've pushed 3500 ppm to autoflowers, twice a week and they loved it, I've never had a nutrient burn issues. I would say each strain or hell even each plant will eat different, just slowly add more until you feel like you're feeding them what you want. I've spoken with breeders who have told me they regularly run 7000 ppm during flowering. Experiment and find what works for you. As far as ppm and pH meters I would recommend an Apera PC60. Happy Growing!!