I was using a quarter of a tablet for 5 gallons pre-boil. I may try RO water on this next batch and possibly a smaller batch with tap water and leave it out over night with the campden added in.
RO water is perfect, especially for light colored beers like Spotted Cow.
I have great tasting water, but it's got a high bicarb level due to where I live so I had to switch to RO water myself for the light colored beers I like to brew.
The only beer I can make well with my tap water straight is stout! Even then, my mash pH is 5.56 or so, with the blackest most acidic grains. Making a light colored beer, I've measured a test mash as high as 6.0 with my tap water.