I'm at the 8th week (day 50). When do you adv...

I do a flush just before flipping to 12/12 (you're past that though), and a flush two weeks into flowering (as the flowers begin to show, you're just past that). Looking at your plants though, they are not showing any signs of deficiency or excess - hurrah! In the happy state they look in - I don't think you need to flush just now, just switch to Bloom nutes and think about flushing two weeks from when you plan to harvest. By not flushing now you will keep some Grow nutes in the media (Nitrogen) which they will like a bit of during early Bloom which you're in now. They are a bit stretchy. I put that down just 250w of light in flowering, compared to the 600w you did in veg. They are going to continue to stretch for a bit, so consider some LSTing so you don't run out of space. Another side note is that your solution temperature is too low and will cool down the media and hence stunt growth. Make it ~22-24c going in to not stress her with cold feet! :)