Almost identical observation here using similar medium, I've run from 500-650 and noticed significantly more tip burn at 650ppm, 600 seems to be a happy balance for most strains I've run in this type of medium. I've seen people mention ppm's in the 1000's and to me it seems like a waste of money and make-work program dealing with associated issues.I usually hover around 600-700ppm during flower. Only a bit lower during veg. I've tried pushing all the way up to 1200-1300, and wound up with the same results as I did running at lower strength. The only difference was I had to deal with burning, and lock out issues.