Need advice for 3wk old seedlings ebb & flow

What's Riu...Well i have 5 R.K.S seedlings from dna genetics,i started the seeds under a 216w cfl for 2 weeks,after that i put them under a 600 watt mh 24,"away from the top of the plants..i started feeding them pure blend pro grow,calmag plus and aquasheild,at 300 ppms,but then i backed the ppms down to 250 ppms because i burned them a little,i tried to feed them today at 1/2 stranght 500 ppms but they burned again just a little,not too much so i'll bump the ppms down to 380....At present time the light is 20" away from the tops and the temps are 70-78f...humidity is 45-50...water temp 60-70...ppms are at 380,and i flood the tray once a day....My question is.. how close should the light be to the plants? and is 380 ppms ok for now,?and is the water temp ok?..I've grown in ebb & flood before,but i used clones,not seeds,and it't driving me crazy lol..I gotta be doing something wrong bcause the plants are 1"1/2 tall..Sorry for the long post,and thank you for your advice..1 luv