Percentages are nice, but dollars are what go in the bank. You can not make these decisions without sales performance in the equation.
If one special produces 100 orders and the other produces 30 I will take the one that procuces 100 orders regardless of the percentages. In my 14 years, I must have tried 100s of offers. Some have been real turkeys despite having good profit potential or being really good deals for the customer.
If you want to dig into the numbers, you could look at gross margin productivity of the offer taking into account unit sales, food cost and price combined. Leaving any one of those three out makes the result meaningless.