Ever seen this before?

So, the smallest of the 3 plants, is producing the larger buds. I am about 3 weeks into flowering. However, this rusty yellow started appearing on the of the veins of the leaves. I have no means of testing the pH at this time, but its been recently mini-flushed and given diluted bloom nutes once. No mites. And no idea whats wrong...

growing in soil.
under 400w HPS 12/12
1-3-2 nutes given

I know there's a 'plant problem' thread, i've tried everyone, no ones gota anything :/

looks like Mg deficiency.

ph is off..prob low, kinda looks like calcium deficiency

Im looking at my Handbook: Marijuana Garden Saver, By Ed Rosenthal: (www.quicktrading.com) Highly recommend for everyone. It has helped me alot! And the pictures are great! It looks like PH, and it reads "The only accurate way to adjust the ph is using a meter or ph test papers. Guesswork won't do. PH 6.0-6.5.
Also the r pix. of CAL & Mg. deficiency, which looks fimiliar. And to be on safe side u could add cal-mag. see if helps. But if PH is not adjusted the nutrients will not dissolve well, which r not available to your plants roots. PH meters are a must for any grower! Peace & Happy growing.