I use the 3/4 in with foil. If you have been over feeding and (I use 800 to 900ppm in coco)was for long time and in dirt or a blend of most medias there will be a build up in the mix.
You have decided to flush and you will us the phed water to flush. The nutrients will be released all at once and make everything worse.
In your situation and ask others but I would cut by half next water by half again and do that 3 times if you have enough time left till axe. The end I would drop to no nutrients and a pinch of sweet.
There are others here that use your mixture they may doo it differently.
I use Jacks and grow in coco 1/2 is a reuse after washing and screening and 1/2 new.
One of the ways to tell how much nutrients you have used is there any white residue on the top of the media.
I use metal skewers from BBQ pit and I loosen the media at the top and down till you touch the roots stay away from the roots closes to stalk.
In the day when I grew in dirt in 5 gal bags. I used enzymes to dissolve the roots that die off was a canna product and works well. When the roots are in a confined space some die off and they occupy space and are stuck there if you use an enzyme they are dissolved and new roots replace them.
In time you will let the ladies tell you what they want but by forth week in flower your time is limited this is the time I am adding
moab.
I have read many and used Gt21 tips in the past most here will answer questions but it has been a long time for me and pots of dirt.
Weed don't like wet feet and I add this in I have no idea if will help you but never leave the bottom of your pots standing in water. For more than a short time. Why? the water evaporates and the nutrients slowly build up and become toxic.
After all that some of the strains are hard to grow and require changes in nut lvl's and additives the white strains love C but if you use Cal/ mag the C is good and the mag is in excess a little but the added N will put you in N tox.
I grow in a confined space and is hard to keep heat under control and I learn cooler is better after getting new a/c most of my problems have gone away my light bill went up. When I started out growing with led's now I use both. In time I learned that more smaller lights was better than 2 big ones less heat and the smaller light don't over heat the leaf structure.
You will learn to adapt what you read to what is happing.