There really is no such thing as the ideal tap water TDS going into an RO membrane. I can tell you the national average for TDS is about 250 ppm TDS. Some parts of the country have lower and others much higher like here in Phoenix where it is about 800 most of the time.
I am not sure what you are trying to accomplish since prefilters and carbons have almost not effect on TDS. They are there to filter out TSS, Total Suspended Solids or silt and sediment for the prefilter and CL2, or chlorine residual disinfectant and VOCs or volatile organic chemicals. TDS is total disolved solids or things much smaller than a prefilter will filter out. Adding additional filters will not help TDS in any way.