Thought my problem was fixed - looks like it's not - please help! (pics)

Last weekend I posted about a problem, which seemed like was caused by nute burn. On Monday, I flushed my plant with water 3X the size of the container. Until yesterday/today, everything seemed fine (no new problems with any leaves).

The new problem is in the pics in this post, and is on a leaf at the very top of the plant (the previous problem from last weekend affected bottom leaves only). Note, this problem is starting from the middle of the leaf, NOT the tip, which makes me question whether or not it's a deficiency or something else. From what I've read deficiencies start at the tip of the leaf... but, I could be completely wrong.

That being said, does this look like magnesium deficiency, or could it still be nute burn? I haven't watered again since the flush, as she's still not dried out.

Here are additional details on the grow history:

Grown in Soil.
How long has this problem been going on? - 2 days, but see above for detail
What STRAIN are you growing? - Sativa (unsure of exact strain)
What was the establishing technique? (seed or clone?) - Seed
What is the age of your plants? - 13 weeks
How long have they been in the soil mixture they are in now? - 12 weeks
Were they in the same mixture when they were seedlings/smaller plant? If not, what mixture were they in before? - They were moved to this soil when transplanting from small cups at 1 week
How Tall are the plants? - approximate 46 inches
What PHASE (seedling, vegetative or flower) are the plants in? - Just finished 4th week of flowering
What brand Nutrient's are you using? - I'm in Europe, so the brands are different.
How much of each nutrient are you using with how much water? *Knowing the brand is very helpful* - I was using a mix with 4% Nitrogen, 6% Phosphate, 6% Potassium Oxide, small amounts (less than 0.01% of each) of Iron, Boron, Copper, Manganese, Molybdenum, & Zinc. Using 5 ml of solution for every 2 liters of water. I just bought a new mix of 15/30/15 (NPK), but have not used it yet.
How often are you feeding? - once every 5 days or so (when the top few inches of soil are dry)
If flowering, when did you switch over to using Bloom nutrients? - first watering after switch to 12/12, which was 3-4 days into week 1 of flowering
What is the pH of the "RUN-OFF"? - Soil pH is 7.
What method of pH test was administered? Using Strips? pH pen? - pH meter used to measure soil pH, when soil is wet.
What is your RH Factor? (Relative Humidity) - Low humidity
What is the Day/Night Temp? (Include fluctuation range) - 75/68
What is the current Air Flow? (cfm etc.) - growth area is large, no mechanical system in place for air flow.
What water are you using? Reverse Osmosis (RO)? Tap? Bottled? Well water? Distilled? Mineral Water? - Filtered through Brita
Has plant been recently pruned, cloned or pinched? - No
Have any pest chemicals been used? If so what and when? - None
Are plant's infected with pest's? - No

Thanks in advance for any help.

that to me looks like a phosphorous deficiency. check this out. someone posted this once and i saw it and saved it. so here you go.



that new fert you have should help with this. I didnt see that you had it even or didnt use it yet.

What lights and how close are they?

Symptoms of deficiencies manifest themselves where ever not just at the tips. In fact rarely at the tips.
Quote"What brand Nutrient's are you using? - I'm in Europe, so the brands are different.".... No they aint. nutrients are global.
Wait longer to switch to bloom nutes. You'll keep more leaves later on.
After flushing did you go straight back into Full strenghth nutes ? Advise normally that you start back with a half strength or weaker for up to a week. This is the part that helps correct the deficiencies especially if the build up of one is locking another out. (I'm taking about the nutes in the plant not in your medium.) Going straight back to full strength can sort 1 issue and create others.

Lights are CFL's, warm white, they're not close enough to burn at all. I'm pretty sure now that the pH of my water is too low, so I'm going to use distilled water going forward. When I mix in the nute mix I mentioned I'm going to start using (15/30/15 NPK), I'm going to add a bit of baking soda to keep the pH closer to 7.

Does anyone have any idea how much baking soda is needed to alter pH in a liter of water?

I don't see what strength your nute solution is. If you're not keeping track of that it might have been a one time thing.

Get a Ph meter or test kit.

that's a calcium deficiency - sprinkle your soil w/ a couple of tablespoons of dolomite lime. takes a week or two to take start working