Just to add to that ronnie, to give you some idea of your expected resin consumption-
My tap tds is 96ppm, i run a 400Ltr van mount, BUT- I estimate i'm about 75% wfp & 25% trad, i average 4 days per week & With the above method i use 2 x 25kg bags of resin per year.
I use dowex from wintecs @ 81 quid inc' vat & delivery, so my annual cost is 162 quid.
tony