Unaltered, my water from the tap has ~ 4x the recommended 35 ppm CaCO3. If left unchanged what perception will that have on the finished beer?
It depends on the beer. It might work great on a stout. Pilsners will have a “muddy” finish that lacks that crispness they should have.
A friend has water with similar alkalinity. His Pilsners improved when he blended the water with RO. He didn’t say what the blend ratio was.
A straightforward approach to blending is just to dilute until nothing is above the desired limit, and if that takes the calcium too low, add some back in. I’ve successfully used this approach on Pilsner. But I must say I now am happier building all the way from RO, because I’m finding the SO4/Cl ratio has a big effect on Pils. You might still be able to address that by choosing your calcium salt.
I’m in the same boat as the OP when it comes to tap water. Like Robert, I find it easier to just go all RO/distilled rather than diluting. Buying water is more expensive, but has the added benefit of letting you not worry about your tap water profile changing from what was tested.
My tap water has ~650 ppm, 364 ppm CaCO3.
You can guess what I do.
That statement isn’t telling enough. Both hardness and alkalinity are often quoted in terms of ‘as CaCO3’ and your description doesn’t clarify that.
In other words, nobody can assess what that parameter might mean for your beer.