If you decide to make the white velvet cake, Rose notes on page 483 that a single 12-inch layer is just the right size for 40 servings. You’d make the white cake base on p 491, multiplying ingredients by 3.5. The baking powder chart is on the next page. Multiply the level 3 amount - teaspoons, ounces or grams, whichever you prefer to work in - also by 3.5.
I made the white chocolate whisper cake a couple of times in the single layer 12” size. It was years ago, and I didn’t know enough then to worry about the baking powder. Dumb luck but I just baked the full amount of the recipe from p 50 in a single 12” pan. No problem! I got intrigued and did the calculations just now to figure out why. You increase the bp to take it to a 2” high pan, and then decrease it to go to the larger diameter. Bottom line, it stays the same! The other ingredients could be increased slightly, but the recipe works just the way it is for a single 12” layer. I used it to make a larger version of Blueberry Swan Lake, p 165. A huge hit!