From the eariler discussions, Stu sugested 5g per 20 liters ... which brings the hardness of the wellington water up to that of Dublin.
I had a play with additions last night and it actually very hard to balance the amounts to get the water exaclty right. Best just to aim to get the one part of the water right that makes the bigest impact on the style.
I didn't ask about how ph balancing would affect it all.
so, does the wellington water need to be raised or lowered in general?
also, it sounds like depending on the grains being used, we need to add more, to either raise or lower the PH.
how much effect does the grain have on PH during mashing?
which grains have more effect on PH levels?
and on which types of beers is it more important to use them?
Don't get too hung up on water. There's plenty of other things to worry about first.
However.... in general, all over Wellington (and probably NZ I imagine), you will always need to acidify to get the right mash conditions.
Acidifying will let you hit mash pH but will not alter the mineral content of your beer to much. Adding minerals, even just a gram or two, can drastically effect the flavour and aroma of your beer. Beware.
Palmer reckons otherwise. 10% roast might not bring it down enough according to this:
When you mash 100% base malt grist with distilled water, you will usually get a mash pH between 5.7-5.8. (Remember, the target is 5.1-5.5 pH.) The natural acidity of roasted specialty malt additions (e.g. caramel, chocolate, black) to the mash can have a large effect on the pH. Using a dark crystal or roasted malt as 20% of the grainbill will often bring the pH down by half a unit (.5 pH).
Adding anything alkaline would drive those acidity gains back.
There is one good way to check for sure - buy a pH meter.