Don’t even bother wasting your time trying to work it out , or even experimenting using resin with a tds that high . You are stuck using yiur RO . My tds hoes from 70 up to 300. When it wants . Summer , winter , changes week by week , doesn’t seem to be any pattern . I just use the RO all the time now . Brings it down low enough to not use resin . Usually down to 3 up to 12 . Just gotta take the hit with my water bill for the time being . Having an extension and new roof , starting in January. When thsts done I am definitely gunna set up to harvest as much rain off the roof and garage . Water bill at the moment is outrageous.