Only done an RIS and that's got about 8g per litre of roasted barley, it's as roasty as roasted toast! Really very roasty in other words, it's mellowed into the overall flavour nicely but remainins very much the dominant flavour.it would IMHO be too much in a standard stout. Other people may have different experience but from mine I'd be reluctant to go much higher in future
No idea really but the acidulted malt as far as I can tell will lower the PH when you already have alot of dark malt making it already acidic, is there a reason for it?
The acid malt is to give it a twang I believe. Not sure if I noticed the difference. I have used up to 15% roast barley before and made a decent beer. I cold steeped 10% and used 5% in the mash. You need a high attenuating yeast though if you want it dry.
My first stout was a chocolate stout but I found it too bitter/ choclatey for my liking.
I've done many stouts since and it may have been me that suggested the acid malt. Yes it does give a twang but my stouts haven't used roast barley or chocolate since. The acid malt gives the twang but there could possibly be too much twang in this recipe.
Proportions look great, stouts should contain around 70% base malt, 20% flaked and 10% dark/ roasts. I'd try it and see especially if you like the roast, chocolate flavours.