No, campden doesnt really change it. 1 tablet per 20 gallons is such a small amount of sulfite, you can ignore it.
The only caution of trusting bru'n water without a pH meter is that different malts can give different responses. For example, rahr malt tends to give you a bit lower pH. If you stick with the same malts, then it's just as accurate as loopie says, but you can run into some variability if you switch base malts often. And I'm not talking about a lot. Does it matter? Maybe, maybe not. Depends on the beer you're making.