How to determine the capacity of waste/number of units required to cater efficiently for the number of covers served in a catering facility?

The calculation is more a rule-of-thumb as each establishment will differ. Usually outline the following: Use a range of 0.2 Kg to 0.4 Kg PER COVER, the lower being where you think there is good control, the latter where it is a more expansive menu i.e. more wastage.
For example a restaurant does 100 lunches, 50 breakfast and 250 evening meals, with a total of 400-covers, and it is a broad menu, we have 400 x 0.4 Kg = 160 Kg approx. per day.