Need help figuring out whats wrong

Problem started about three days ago, I have eliminated spider mites and powdery mildew as the potential problem. The damage started as tiny white dots on the inner veins of the leaves, progressing to the point now that the affected leaves are pretty well covered.

I am using a pretty new blue labs PH/ppm tester and have it calibrated regularly.
PH of water has been going in at 6.5
PPM last feed was 800, using a mix of sensi A+B, big bud, bud candy, and cal-mag due to our soft water.
White Widow, day 50 in flowering.

At 50 days into flower, I am inclined to say this is not going to be a problem here.....I would begin by removing those fan/shade leaves as the yellowing progress's. (This will also allow more light to the buds sites, thus allowing a more uniform and timely finish).
Might be a tic high P as the AN does that..Skip a "feeding" and use plain water, it may help slow it down. But like I said it's more natural at this point for the plant to do that......It's "using the available "nutrition" in the plant from now "unneeded" vegetation.

They really will finish out just fine from this point...

cool thx for the tip.

Yellowing i get at late flowering but the white dots without any bugs was confusing. could not find anything about it when i google it all i got was spider mites