Several authors have argued for the role of reading code in programming education. The literature suggests e.g. cognitive overload as a significant problem and discusses the need to divide learning programming in sub-tasks, and to explicitly acknowledge and train different aspects of reading skills, e.g. reading for general understanding, tracing, and debugging. In this paper, we present a theoretical analysis that results in a tool design that correlates observable gaze behaviour to a theoretical model of source code comprehension. This allows to link observable reading behaviour (gaze) to internal cognitive processes of source code understanding, and thus to research the effects of different reading tasks and approaches on learning.