If using RO water there is no EC value in it as the filtration has pulled all dissolved solids from the water making it TRU H20. Sorry been doing alot of reading on RO water as that is what i have ran all grow so far and is getting quite pricey, lol. I am investing in a RO system at TAx time. The ratio for PH Down per 1gl that i have found using RO water is literally one drop per gallon(PH Down)-GenHYDro, with no EC value a very little goes a long way. PH UP ive found is 3 drops per gallon after adding ph down to raise back up to 6.8ph-FOR SOIL. RO Water starts with a ph of 7.0-7.2 so with soil 6.8 is perfect water ph. Find a chart on the nutes your using and see what the ppm is in general with their nute line and and add 10-20ppm for RO water to that just to be on the safe side. RO water depending on quality of system used can range from 0-20 starting PPM with 0 EC. all that will change when adding ph down or up and the nutes that youre using. Hope that will help. If not just tell me to shut up, lol.
Also after looking at the pics, in my opinion, they are eating all the Nitrogen out of the leaves and they still have 41/2 to 6 weeks depending on strain. I would do a little research first but i would recommend bumping up the Nitrogen ratio of your feed mix for acouple more weeks and then let them yellow as they will be on the last stretch of flowering out. I would respond quickly before bud production comes to a finish early. Just my opinion, im not an expert just did a lot research and trying to offer helpful tips i rememeber. Good Luck Bro!!