I use this malt all the time in my Dark Mild..... I brew it about 10+ times a year. The one thing I would say is it is a "sweet" malt. When I first tried it, I brewed my mild and used it for all of the base malt (about 75-80% of the total grain bill). I thought it was too much - too sweet. Basically, I now use:
45% Maris Otter
35% Ashburne Mild
20% Specialty Malts
I think this is about right for that beer. I have not used it in anything else, so I can't say for sure how it is in a stout. But, you can basically go two directions with it - use "too much" and scale back on future recipes or use "too little" and add more in the future.
If I was guessing, I would say less than 50%, more than 25% if you want it to impact the beer in a positive way.
Kegged/Serving: Citra IPA, Oatmeal Amber Ale, American Wheat, Ordinary Bitter, Double IPA
Bottled: Barley Wine, Sour Belgian Golden on Cherries, Sour Cherry Stout
Primary: Heady Topper Clone, Double Citra IPA, Munich Helles
Secondary/Lagering: Schwarzbier, Oktoberfest, Bourbon Barrel Porter
Future Brews: Bo-Pilsner, Porter, APA, American Brown, Oatmeal Stout, Schwarzbier #2, British Dark Mild, More...... More....... Beer.......