At one time, I suppose this would have been the default malt for making a roasty stout (or stout-porter, or porter, or whatever it was called in that era).
I have used brown malt as part of a recipe inspired by Radical Brewing. It used something like 4#s of brown malt in a 5 gallon batch to supply roastiness. If you are using flaked barley in your dry stout (which is common), don't look to the brown malt to help you convert.
Just off the top of my head, I would guess that you might start with something like 20% flaked barley, 40% brown malt, 40% pale. Try something. You might need to tweak it to get the right texture and roastiness.