Test your water after the ro part and before the DI. The DI reduces the TDS to 0 until it is exhausted. You don't need any other filters to do that. Problem is DI costs a lot so u only use it to give the water a final polish.
Your RO filter should bring your water to at least 98% TDS free.(actually varies, depending on the gpd figure of your ro) If you start with 25ppm it should read 0 after the ro.
Your prefilters will take out the particulates first, then some dissolved solids.
Your TDS meter will only read dissolved solids, not particulates. I do not believe any dissolved solids will "sneak" past the meters ability to read them.
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