The question that raises is how long is the add back method good for? Without actual lab testing there is no way to know. Since the plant uses varying amounts of X Y and Z the very first add back the quantities in that PPM are going to start changing. If you ran lets say for a month, 2 months, that 1250 ppm is going to be quite different in its composition of XYZ. Which in turn usually leads to greater PH instability which I personally only want to PH water once. I think the more PH up or down you add to it isn't good over time either. Basically it will get to a point where the PH won't stay stable for any amount of time and your mix while although 1250ppm is going be vastly different than a fresh batch of 1250ppm solution. Then you might start to see plant issues from either deficiency of X or excess of Z. I don't think the nute savings are going to be THAT substantial to try and push the envelope so close but I just wanted to throw some more stuff out there to fuck with you.
In "my" world when I would run a recirculating system I mix it once, top it with fresh PH'd water as needed and once I had added back the volume of the res in top offs, (i.e. 20 gal res, 20 gal added back) I drain it and start fresh. The plants are already growing so god damn nice and faster than a soil container I'm more than happy to be satisfied with it and it couldn't get much easier to maintain. I also never look at the res PH other than the first mix and the PH of the top off.You may have already addressed these things, I see you are a lot better with math than I am and I haven't got all the way through your thread yet so take what I say with a grain of salt eh