If feedwater is 70 ppm, the concentrate (aka "waste water) TDS would be HIGHER that 70 ppm.That's something I've never really thought about before but my guess is that it would actually be less than 68 ppm since some of the impurities would get captured in the carbon blocks and in the RO membrane itself.
If you're thinking about reusing that waste water I would let the system run for a while beforehand since the waste line will contain come carbon dust in the beginning, but I guess it depends on what you're using it for. I know that after a carbon block is exhausted, it can start dumping captured impurities back into the water and the TDS might actually go up but that would take a while.