Design teams commonly form quantitative functional specification lists that define performance targets for a product. Meeting these specifications ensures that the customer needs are satisfied. A central difficulty is to identify the relevant metrics to use as specifications. A working methodology is presented here to establish relevant, quantitative, measurable performance specifications. To start, customer needs, their importances, and the customer use patterns are gathered for the product. Voice-of-the-customer methods are augmented to distinguish no-compromise constraints that must be met. Next, a design is analyzed to establish a representative function structure, applying Pahl and Beitz’s systematic design approach. An optimal function structure can now be defined by a condition of simultaneous maximum simplicity and comprehensive coverage of the gathered customer needs. The function structure can then be used as a list over which specifications are to be made, with at least one specification per sub-function. Having a rational function structure allows a team to more easily determine variables on which to make specifications. A team can then use the House of Quality to document and form consensus over these specifications in the typical way.