Cameron,
There are three main types of membrane fitted to 40inch RO housings. HR (High Recovery) HF (High Flow) UHF (Ultra High Flow), in general these membranes will reject 95-98% of Total Dissolved Solids. Therefore if your source (tap) water has a TDS/PPM (Parts Per Million) value of 100 then you should expect a permeate (product water) value between 2 -5 TDS/PPM from the RO. It follows therefore that if the source water quality is 300 PPM then the output permeate reading will be in the range of 6-15 PPM, if 400 then 8-20PPM and so on.
Hope this answers your question.
Kind regards
Craig Mawlam