Waste for an R/O can be anything from 3 parts waste to 1 part pure to 50/50 and above.
The 4040 R/O's are the most efficient and fastest producing ones, the RO man ones, (200 gal per day and so on) are the slowest, but to be honest, they are more than fast enough (even though the waste to pure is 3:1) for a one man or even 2 man outfit, especially if you are just starting to build your round.
I used one for 4 years and the last couple of years I have been keeping 3 of us in water without to many problems.
If your water is 0.115 ppm then you will most certainly need an R/O system of some kind, a DI just won't be cost effective over the long run...
Ian