Hello Steve, well funny thing is mine is not zero at the moment, I've got to get new membrane di resin also. I do aim for 0 ppm. For aquatic use as I like to know there's nothing going into our crs sbs tank other than me remineralizing with salty. anything that comes out upto 8ppm I use for window cleaning or mirrors around the house, shower doors, tank glass etc. , to answer your questions yes if everything is spot on you should have 0ppm. Check that you di resin is packed in tight, after a few days running I get a channel along the top of the resin by passing the resin rasing the ro water that comes out to as much as 12 ppm.
I add a bit of fresh di, repack then it lowers it to 0ppm.