Buds Growing Great But Fan Leaves Are Dying!!! HELP!!

I am 4 weeks 3 days into flowering and my fan leaves close to plant tops are curling up and becoming dry and brittle,yet the beds are growing great any ideas?

Soil: FFOF with added perlite and Light Warrior mix
Water PH: 6.5-6.6
Soil PH: 6.7
Runoff PH: 5.9-6.1 on all plant
Soil TDS of Runoff: 360 ppm this time watered with 200 ppm R/O with Molasses
Light: 600 watt HPS batwing reflector at 22" above canopy
Temps: 73f lights on 68f lights off
Humidity: 40%
Feeding PH and TDS: every other watering 500ppm 6.5 PH of 2ml Calmag 3 ml Liquid Karma 8ml Sweet 5ml Pure Blend Pro Bloom

 

These are Blue Mystic an Northern Lights from Nirvana the two worst ones are both NL

 

final few pics to show bud development and the leaves dying

 

It was only 500ppm of nutes though??? And 5ml of pure blend pro bloom was 50% strength. So Should I go for less than that like 250 ppm??

 

So what can I do now? I have 3 1/2 weeks left of blooming if they live that long . I have been using a super weak nute solution I don't see how this happened.

 

Another forum is saying its classic deficiency with the leaf curling?? So I took the wrost two and gave them a fertilizing.

 

If runoff is 5.9 what ph water do I want to flush with 6.9?

 

Should I not resume any feeding of nutes for the next 3 weeks after the flush?

 

Thanks bro,I feel it is potassium and phosphorous deficiency for sure there is no way at 300 ppm runoff my plants are burnt.

 

Thank you soooo much this is how I was looking at it too,everyone on here says all problems are nute burn lol. I'm going to get the other 3 with a 500 ppm of just flower nutes in the morning.