I would center in on the ph ... my white widow likes to have the ph more on the low side. Some of the micronutrients do not absorb at the high end of the 6.3-6.8 range and it sounds like you are trying to shoot for 6.8. You might have better luck shooting for the middle of the range instead of the upper edge. I try to ph everything to no higher than 6.45 and it works well for me.

The brown tips, that is just an indicator that you are giving just enough nutes... I have heard it said that if you don't burn the tips you are not trying hard enough. I am looking more at the inconsistent coloring within the leaves...