Leaching your soil with copious amounts of water is a good idea a couple times each cycle as long as you have good drainage. It will help solve nutrient lock-up which occurs when your plant gets too much ferts. Nutrient build up can easily burn leaves and leave them dry and brittle.

Moving a 400w light to 24" sounds excessive. Remember that light decreases to the inverted square of the root. So.... 2 ft away only give 1/4 the lumens that 1 ft away give. 3 ft would give 1/9 the light. I use a 400w MH for veg (which gives 40,000 lumens at 1 ft, why settle for 10,000?) and I keep the light 8-10" and have no problem. My room temp is always 78 with lights on. No Probs!

Heat burns the crowns first while nutrient problems usually start from the bottom. Hope this helps!