When a crack propagates at a heterogeneous brittle interface, the front may be trapped by tougher regions and deform. This pinning induces non-linearities in the crack propagation problem even within Linear Elastic Fracture Mechanics theory. Predicting the crack front morphology from the local energy release rate distribution, and also the relation between toughness pattern and average energy release rate is a challenge. Here we have experimentally investigated the propagation of a of purely interfacial crack in a simple toughness pattern: a single defect strip. The quasi-static propagations has been monitored directly in glass samples and analyzed by two different methods. Starting from the crack front deformation at equilibrium, the energy release rate pattern has been calculated directly by the Finite Elements method. Analytical perturbation methods were also used to predict the toughness contrast. We show a quantitative agreement between the two methods when the finite thickness of the specimen is taken into account. This opens the way to a more accurate use of the perturbation method to study more disordered heterogeneous materials, when the finite elements method is less adequate. From our results we also propose a simple method to determine the adhesion energy of tough interfaces by measuring the crack front deformation induced by known interface patterns.