I have to do a stacked book cake this weekend and I'm not sure how many books to make. She wants the cake to feed at least 75. I can make them one layer or two layers (or a combination of thick and thin)and I'm not scared of stacking tall cakes, I just don't know how many I would need. Do you have a suggestion? I've thought and thought, but my mind is blank. Would a 9X13 be the best size/shape?