Questions about yellowing leaves and flushing

my top leaves are all turning yellow my bottom ones are not. I'm guessing this is a nutrient lock out problem? Or maybe because my humidity dropped down kinda low for a few days (30%)? so going off the little knowledge I have I was going to go ahead and flush but I'm scared to go three days (soil drying period) with out feeding again because the top leaves on the top half of the entire plant are almost completely yellow. Should I feed again right after flush or wait the three days for the pots to dry out?
 
how far into the grow are you? what are you feeding it? whats the ph?
Im about two weeks out from harvest judging by the trichroms. I'm using a salt based nutrient line from my local grow shop ph of the soil is 6.3. I went ahead and flushed with straight water and I just found the problem. My ppm' are way way way to high lol at 1900ppm's ph was fine still some how at 6.3. So I'm just going to go ahead and let the dirt dry up before I feed again unless someone tells me otherwise. I'm a newb and can use all the help I can get.
 
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