I’ve started using distilled water and building from scratch; it’s not that hard, a bit confusing at first.
Stout water and porter water are going to be close but still a bit different. For example google a water report for Dublin for a stout and for a porter google a London water report.
You can use ProMash’s water analysis if you have it; they’ve got built in water reports and a water recipie calculator. Don’t worry about matching the water perfectly it’s almost impossible. Just try and get close and keep the ratios close.
Few things to keep in mind:
Grams don’t convert well to tsp/tblsp so be sure to see here for conversions How to Brew - By John Palmer - Using Salts for Brewing Water Adjustment
note the grams to level teaspoons
Add your salts to the mash and mix in.
Note the volume of your first runnings.
Sparge and bring up to finale preboil volume.
Subtract the first running volume from the preboil volume and add enough salts to make up the difference. For example = 7 gallon preboil volume - 3 gallon first runnings = enough salts to compensate for the 4 gallons.
Add the salts before the boil starts. I once added salts for 3.5 gallons directly into a rolling boil and it foamed up and over the kettle.
Hope this helps!