Bottom line, of course, is if the bride wants to buy lots of extra cake, it's her money, go for it.
I ask a bride "How many guests are you expecting?" Then I ask her "How many did you invite?" The reason I ask it in this order is because it shows me what kind of bride I have in front of me. A bride who is expecting 250 out of 300 invites is a little overconfident about her popularity. A bride who is expecting 200 out of 300 invites is dealing a bit more in reality.
I explain the 60% Rule. I then suggest "you can have a 4 tier square or a 5 tier round, basedon those numbers". I then figure the cake sizes (based on my stick-people-cakes that I have in excel) and we quote based on the number of servings the configuration adds up to, based on the Wilton Wedding Chart. I also tell them they can increase or decrease the amount later, depending on what cake sizes we can swap out.
I think she's spending too much money to assume EVERYONE will eat 2 cupcakes. A cupcake is a LOT of cake. But ..... it's HER money!