This paper presents an approach to designing interactive systems that enables critical performance parameters to be identified and models of performance to be constructed. The methods described are intended to enable designers to improve the performance of systems, and the provision of performance targets is expected to encourage innovation in design. An example is quoted in which digital camera technology was applied to the support of authors using paper source documents, to enable them to capture source text more rapidly and thus increase their productivity, measured in terms of words per hour. A model of the capture task was constructed, and was used to set a target time for capturing short text segments. This target was presented to a design team, who responded with an innovative interface incorporating autocompletion. A prototype auto-completion tool demostrated that the performance target could be met.