i'm not sure about the whole acidulated malt in stouts thing. Basically what youre doing is just lowering the pH (making it more acidic). Dark grains already make the beer acidic. I think it was originally based on the guinness idea of adding 3% soured beer , but Dublin water is highly alkaline and full of carbonate, so the beer could deal with some extra souring.
When I make stouts with Sydney water, which is soft and low in carbonate, the dark grains will often push the pH way down and make the beer harsh and acidic and thin. So youre starting with a much more acidic beer (than Dublin stout) to begin with. Adding acidulated malt will just worsen the problem. Unless of course you chuck in heaps of chalk to simulate the Dublin water.
What it is good for is if you're making allgrain pilseners and you don't want to add minerals to the water, 150g of acidulated will get you in optimum pH range.
You can easily make your own, just get a one litre thermos, mash 200g of pale malt grain at 65 for an hour, let the temp drop to about 40-45 then sprinkle in a few dry grains, close up your thermos and let it sit overnight. (Longer if you like but overnight works) Should be nice and smelly the next day and will probably drop your overall mash pH by about 0.2-0.4. I did this for my last pilsener and it worked perfectly.
Good luck
N. meate
