I think ideally RODI
is the way to go. I've been using RO
water from one of those big dispensers in front of the grocery store (Glacier). Sometimes I top off with distilled that I get in the one gallon bottles inside the store.
I just got a TDS
meter for $11 and have started testing everything just to see how it all compares. The distilled store-bought is 002. The machine puts out .004. I've tested a couple bottled drinking waters just for fun and forget exact numbers, but I think Dasani was .003 and Aquafina was much higher. Did tap water and refrigerator-filtered, which were both much higher too, but can't recall exact numbers.