i know, it seems like a basic question - but if you are quoting a 100 person 3 tier cake for example, is that taking out the top layer (for the bride and groom to take away) so youre only counting the 100 person servings in the two layers?

http://cakecentral.com/gallery/1197485/montaje-de-bizcocho1

so for example in the picture, the one on the first row on teh right, says 116 servings - does that count in the 116 the top layer or do you minus that out...?

I would just say, this cake will feed 100 people including the top tier, or 88 (or however many) if you're not planning to serve the top tier.

Then charge for all of the servings

I charge for every serving in a cake.

Whether or not the couple is serving the top tier or saving doesn't matter to me. I am still baking, filling, icing, decorating...the tier.

I ask if the couple is planning on saving the top tier. If they say "yes" and have 100 guests coming I will sell them a cake that feeds 112 (or whatever the number of servings is according to the closest I can given sizes). I tell them why I am selling them "extra" cake so they understand that 12 servings (6" round cake) is for them and 100 servings is for the guests.
If the couple says "no" then I sell them a cake that feeds a total of 100 people.

Either way they are paying me for all the cake.