You're bang on dude, exactly 25 days into flower. I had started noticing some lightening around the leaf margins about one week before switching to flower, but didn't pay too much attention because it wasn't very noticeable at the time.
I do a flush the week I go into flower just to kind of clean the slate and make sure I don't develop any lockout problems once the plants are heavy into budding. Figured the flush and re-nute would have solved the problem, but it didn't, and like I said, it's only progressing at this point. However, I've never really like flushing more than twice per grow as I had thought it stressed the plant. I guess I'm wrong on that point.
It's obviously something this strain in particular needs more of, because I'm giving a nice balanced feeding schedule, and I've seen this problem on ALL of these plants (pakistan chitral kush - I should have mentioned this before).
I really don't think it needs more nitrogen, as this plant is VERY susceptible to nitrogen claw, and I'm quite familiar with how lack of nitrogen looks, and this just doesn't seem to be it.
She is certainly a picky girl, but I guess that is how some of these landrace strains can be. Probably not really worth the effort, but I just want to solve this problem now because it keeps coming up, and I should really learn how to deal with it rather than just abandon this strain.
I don't feel like flushing and re-nuting will help, because it would have done so when I switched to flower.
From all the troubleshooting guides I've looked at, it seems to come down to potassium or magnesium. I don't have a really good eye for these things, and half the time I'm just going from an illustration, so that isn't the most helpful troubleshooter tool.
I have some liquid cal mag, and I have epsom salts. Should I give one of these a shot and see if it slows the problem down?
EDIT: I'm not using any grow, I'm going by the Lucas formula, and only using the micro and bloom at 5ml and 10ml per gallon respectively. The last time I used grow with this plant, it got nitrogen claw immediately, it just can't handle it.