A true flush is actually 3x pot size. But you are getting close to your harvest window- do you know exactly what the finish time is supposed to be? How crazy do you want to get with this thing? I mean, there's always the possibility of stuffing it in the shower and giving it some rain, right? hahaha too bad your tap pH is so high, but check the runoff first and if it comes up low, like below 6.4, you can get away with that if you make sure that when you water in your fertilizer right after, it's a heavy watering at correct pH. I was initially going to suggest just a nice heavy watering at correct pH for your soil, so that you get a good bit of runoff wihtout doing a full on giant flush. Your call on that one. Have the trichs started to turn at all yet?