The easiest and most fool-proof thing to do would be to make the White Velvet instead, and to use the Rose Factor system in the wedding cake section (be sure to check the errata, the 18 x 12 size baking time is reversed and the data are for just one layer, not two). Those cakes are fully tested and perfected.
If you want to try to make the white choc whisper cake, first you’ll need to multiply the recipe by about 2.25. To get that number, you divide the volume of the pans like this: one 9 x 1.5 round has a capacity of 6.5 cups, so twice that is 13 cups. The 18x12 x2 pan (is that the size of your half sheet?) has a capacity of 29 cups (Cake Bible, p.455). So, 29 /13 = 2.23, I’ve rounded up to 2.25 for ease of measurment since it’s so close.
For the baking powder, first you would mulitply by 2.25 along with the rest of the recipe, so that gives 10 1/8 teaspoons. From p. 492, you can see that to go from a level 2 cake (9” round) to a level 6 cake (half sheet), the baking powder is 1.25/1.333 = 93.8%. So mulitply the 10.125 teaspoons by .938 to get 9.5 teaspoons of baking powder, or 3 Tablespoons plus 1/2 teaspoon. Please keep in mind that this baking powder reduction is for the white velvet and hasn’t been tested for the white choc whisper. It will probably work out reasonably well, though.
So, to sum up, multiply the recipe by 2.25 and use 3T +1/2 tsp of baking powder for one layer in the 18 x 12 x 2 pan. Bake for about 40-50 minutes and be sure to use cake strips on a cake that large.