I’d run those numbers through Brunwater and see where that leaves your pH given your grist - it’s pretty favorable water for dark beers. If you can bump your mash pH up to even 5.5, you’ll make a really nice beer.
Yeah, the water looks pretty good, i think for a stout. You may need to add a bit of baking soda/pickling lime into the mash to get your pH up a bit depending on the amount of acidic roasted grains you are using. My last RIS settled in at 5.6 pH and it is very smooth in the roast department with nice chocolate notes.
Yeah, I normally recommend 5.6 for stouts, too. But my understanding (Martin may shoot me down) is that the mg/l = ppm. If that’s true, bumping up to 5.6pH with 71ppm Na could push him up too far on his Na.
70 ppm Na should be OK for a stout. I’m surprised with the low Ca value. I wouldn’t add any more Cl to the water, but for a big malty RIS, having more sulfate would be OK. The sulfate will help dry the finish out and that is helpful when brewing a high gravity beer. Burton Ale is a big malty beer that was brewed in Burton on Trent and it was highly regarded and it was also reputed to be a forerunner to RIS.
Adding a bit more gypsum to bring the sulfate into the 100 to 150 ppm range could be OK. I would target 100 ppm to begin with and try out a touch more gypsum in your glass of that beer to see if it helps or not.
The Sodium, Sulfate and Chloride look high after entering in to BrunWater, the expected PH without any additions would be 5.58 according to that too. Should I dilute back with say a 1/4 distilled water and add back say a gram of baking soda?