I find this as an interesting reply.
To me what your doing is not a "flush".
To me a "flush" is to put multiple times the pot size in gallons of water through it. Don't do that.
"Start from a clean slate", I don't understand how you could get a "problem" from "any veg nutes still hanging around" as many people continue veg feeding into weeks 2-3 of bloom.
In bloom it is the increased P that needs to be watched. Usually from high P bloom foods and unneeded bloom "boosters".
Running straight water for a time does reduce available nutrition from synthetic nutrients. It should reduce the amount of "stored" P and that can be a good thing in the scope of the plant being able to "use" the higher P made available in high P bloom foods. Of course until the plants ability to store and use the P is surpassed by it's availability, thus creating the P tox problems seen all over this site.
At any rate I find your practice as interesting and your observations from this practice the same. I simply wanted to explain just what "flush" means to most of us.
Peace on dude.