You might want to get some odd sized pans (5,7,9,11 inch) because these help to get two tiered cakes closer to the desired serving size.
You also need to describe your prices so that customers understand that they must buy a fixed cake size (no 9.4" and 11.35" tiers...) and that the servings might be higher than the exact count they want. Or else you can bake the tiers slightly thinner so that you receive the correct payment for the amount of cake you have baked.
But on top of the cake, there is the work which may vary far more than the cake amount. It's fair to have an hourly decorating charge for sculpted and very elaborate cakes.