Here's a wonderful, illustrated link for cutting round wedding cakes:
http://www.cateritsimple.com/id10.html
Here's another chart from Earlene Moore:
http://www.earlenescakes.com/ckserchart.htm
Also, although I personally haven't done any wedding or professional cakes, one of the things I've noticed on forum posts and personally is that if a professional (you) is not cutting the cake, the number of servings can vary to a huge degree depending on the size of the pieces cut.
Costco sheet cakes often have decorations which mark the 2"x2" squares so even an idiot could look at it and know exactly how many pieces the cake will produce.
Perhaps if you make a transparent overlay which shows the cutting grid for each size cake? A grid like this could be provided to the client along with the cake(s) and you could make a "cutting template grid" for any size or shape of cake. (If the "grid instructions" were mentioned in the cake contract, it would also protect you against charges of "not enough servings".)
Hope this helps.
http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm
I use this one.
I use this one too as it is using the industry standard sizes of 1x2x4. If someone else in the industry cuts your cake (caterer or hotel staff) this is how they'll cut it (if they actually know how to cut a cake that is)
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