nut. change

whats up. i'm about to change my nut. sol. right now, i'm flushing the roots with salt leech. they are about 3 3.5 weeks old. last one, i followed the guidlines on the bottle of nut. but the ppm came out to be about 1200ppm, and everyone said that was too much, so this time i'm going to use half of what it says. shooting for around 600ppm is that still too high? even at 1200ppm i did not notice any burning on leaves. i am growing the blueberry strain and some big bud. is it too early to cover a branch on each plant to determine sex? anyone who can give me a quick response within next 30 minutes i'd really appreciate it thanks.