I've run CNS17 before. You can get away with 400ppm (CNS17 and CalMag combined). Results will be as good or better. Weight will not suffer and the plants will be easier to flush properly before harvest. Less salt build-up, and you spend less money on nutrients.
If you don't want to do that, simply add a flushing agent (like Drip Clean, etc.) to your base nutrient every 21 days or so. Flushing agents are just sugar water, and adding sugar to your nutrient solution will accomplish the same thing (i.e. breaking the bonds between salts and the coco). I use Sucanat, but you can use just about any form of sugar, including molasses.
IMO it's completely unnecessary to run plain water through your coco at any time other than the final flush prior to harvest. Lower your ppms and feed every time. Your plants will appreciate the consistency.
No offense to anyone who does things differently. There's more than one way to skin the cat we're all working on. Just sharing my thoughts.