In this paper we consider the problem of designing new elements to be added to an unknown circuit to achieve design objectives such as the assignment of current, voltage or power in prescribed branches. We develop a measurement based model-free approach to the solution of this problem based on the functional form of the variable on the design parameters which are assumed to vary in prescribed intervals. From this analysis we can determine the function, from a few measurements processed strategically. These measurements along with some new extremal results on linear interval systems are sufficient to determine the maximal and minimal values of the current, voltage or power over the hyper-rectangle of design parameters. We provide examples of the usefulness of this approach, along with a verification using PSpice. The paper describes DC circuits in detail but the results are applicable to AC circuits as well as general linear systems. These will be described elsewhere.