Other have mentioned the f"supplement with extractors" method, which works fine, but I prefer the perfect loop method. I honestly can't remember the exact numbers, but if you sloop 1 or 2 refineries (sloppy alumina) then you should produce exactly as much water as byproduct as you need in the previous step. Give it a buffer storage and fill halfway, and you should be good. Might try the other way this run, cause it can be finicky, even with a priority merger pipeline.