I wasn't really seeking fo an answer on "self-rising" flour. By indicating the various amounts required by different recipes, I was trying to show that one would have to have several batches for different needs. We buy flour (and sugar) in 10 kilo (22+ lbs) bags.
Jenny O, can't imagine a country without bags of plain flour.

Sorcha, I looked as well. Google has "recipes" varying from one to two spoonfuls baking powder per cup flour. I checked our old cookery bible, Fanny Farmer pre 1942, and it says 1 tsp/cup may be used as a rough guide. We prefer to adjust according to the recipe.

This brought up the question of the sorts of baking powder used in Victorian recipes. I checked Mrs. Beaton (1883). She says "Already many different sorts are sold.... but the action of all is practically the same. A common recipe for home-made baking powder is 10 oz. ground rice [for bulk, to facilitate mixing], 9 oz. [bicarb of] soda, 8 oz. tartaric acid [cream of tarter also mentioned]." Most people here use "double acting," which is monocalcium phosphate and sodium bicarbonate in a starch to add bulk (takes about 1/3 less in recipes than tartar type).
Did Hobbit cooks argue about this?