I have had this exact question, and as it turned out got help from friends who said add 1/3 cup mix for every box mix the WASC recipe calls for. So, if your recipe is 2 boxes white cake, add 2/3 cup mix from a third box. I tried it last week and it was -perfect-. So, I just have a ziploc back of extra mix that I'll use for WASC.
I had been making WASC the original way, even with the reduced sizes, and it tasted okay but the texture was just...sticky. So the above fix works wonderfully!