First time flowering in rockwool need help

So my first time flowering in rw I have a problem with the tops of the plants turning pale green/yellowish with what looks to be cal/mag issues.

I'm trying to use up the rest of this jacks 5-12-26 and I thank it's got to much k causing cal/mag lockout.

I'm only 10 days into flowering and have not cut out any of the calcium nitrate.
pH in 5.5, pH out 5.8
Feeding 750 ppm twice a day with lots of run off.

What you guys thank?

I began growing in 2000 with rockwool croutons and General Hydroponics three part system.
Occasional forays into other mediums and nutrients always brought me back. Three years ago "Grower's Gold" short chain molecule Humic Acid (Fulvic Acid when short) was added. At almost $300/gallon only 1 ml per liter is used (1 teaspoon per gallon). This seems to help with absorption of nutrients without adding any on its own.
The instructions on the GH3 jugs are used almost exactly, the aggressive growth formula is weakened slightly for new transplants.
pH is close enough to same (5.7 in). Strength starts at EC 1.4 and ends a week later at EC .8.
I do not know where this falls in your system as ppm is either 700 or 500 times EC, depending on reference. EC is EC, same for everyone.

Watering is done from four times to once per day, depending on growth and age. Minimum nutrient volume is at least equal to 1/2 the volume of the container. A seven gallon container gets three and a half gallons, a four gallon container gets two gallons. Under four gallons of medium and growth sucks no matter what so none under four gallons is used.
Side by sides have the seven gallon container growing 15% more yield than the four gallon container with identical clones.

Runoff is 3/4 of water added, less means too long between watering and more means wait longer before watering again. If residue from evaporating nutrient builds on upper layers of rockwool then the chemistry of the container can be thrown off from optimum.

Thanks phaeton. I'm using the matrix disc's.
I flood the top of the pot for a few minutes each watering using high pressure pump.