Brown blotches, w/pics.

I was thinking either salt buildup or Mag deficiency,
What do you experienced growers think?
It started as yellow dotting along the leaf veins and turned into brown inter-venial necrosis

-5 weeks into flowering
-5 gallon smartpot,
-fed 2 to 3 times @ half strength so far since transplanting in veg, gave it a good watering of half strength and has become worse, spreading fast since that last water....
-seems to affect middle & upper fans the most

My guess, Mag deficiency.