Flush is good. Fertilizer burn shows up a lot worse, and faster, in high temps. That's why you are seeing it first on the part closest to the light.
After the flush, feed with a 1/4 strength solution of your bloom fert, and if that is tinfoil on the walls, take it down. If you can replace it with mylar, great. If not, it wasn't going much anyway, since tinfoil is not nearly as reflective as you would expect, and will definitely make your room run hotter. It holds heat even during the early part of the 'night'.