I make my espresso water with distilled or reverse osmosis water and potassium bicarbonate. I bought some RO water at the grocery store and it measured 33ppm. Is that okay? Is that within acceptable limits for RO water? Thank you.

I can't answer that.. but I sampled 4 stores in my town and only the place that had recently installed an RO system had anything below 10.. (there's was 5) and the others ranged up to 150. Since you can't tell what's in the water with a TDS, I only used water from the newer unit. 33 doesn't sound too bad.. but you might have to try a bunch of places to see who maintains their filters properly.

You need to know the TDS of the incoming water to determine percentage of efficiency.
This YouTube link will explain.