Flushing is safe to do whenever, its never too early to "cleanse" the plant with water sans nutes if you are worried about an excess of nutes accumulating in the soil. The yellowing of the leaves can happen pretty early on some strains, its due to the lockout/lack of nitrogen absorption. The same thing happens from inadequate light penetration to lower leaves, and I've seen pH issues cause similar looking color changes. It doesn't look to sever to me, and it looks mainly like a nitrogen deficiency if anything, Phosphorous deficiency looks kinda similar but much more severe and much more brown in my experience. If either are the case though, flushing isn't gonna help, its the lack of nutes in that case causing the problem.
What kind of micro nutrients are you adding, and how are you addressing calcium magnessium inputs if RO water is used? Also, here is a little quote from a strain library pertaining to your strain that might shed a bit of light on the problem, : "Her variegated leaves sometimes curl or "krinkle"; this is an expression of anomalous recessd traits from her diverse ancestors, not a mutation that indicates an unhealthy plant." Looks pretty large and healthy overall, and likely related to nitrogen lockout, its a rather quick eight week strain afterall, and you are nearly half way through the flower.
PS she is 31 days into 11.5-on/12.5-off.
