Water ph a tad high, but likely the soil buffers are still doing their job. 6.3 to 6.8 ph is acceptable range for ingoing liquid ph. (for both, the water and the nutrient solution) Did you get a soil probe or a ph pen?

Miracle grow instructions are usually for a two week period. Go with the low figures recommended, and sneak-up on the maximum dose the plants can handle. I believe I was using a teaspoon per gallon per week. (1/2 teaspoon per gallon, twice a week) Been a while though, so you'll have to double-check if that sounds right per instructions. (too many nutrients used, too crappy a memory)

Did you let the leaves get too close to the light?
Check the undersides of the damaged leaves carefully, and look for little black spots. If you see none with a magnifying glass, then it might be either some drops of water or nutrient solution splashed on the leaves and left to dry, (a no-no) or possibly overdoing the MG. Way back when I used MG, I had better success dividing the feeding days into twice a week at half-dose. And actually...I've never lost that habit regardless of the nutrients I use. (it works well)