The multiplication of communication software based on pictogram grids with voice output has led to the democratisation of this type of tool. To date, however, there is no standard, nor systematic evaluation that makes it possible to objectively measure the suitability of these tools for a given language. There are also no methods for designers to improve the organisation of words into grids to optimise sentence production. This paper is a first step in this direction. We represented the Proloquo2Go® Crescendo vocabulary for a given grid size as a graph and computed the production cost of frequent sentences in French. This cost depends on the physical distance between the pictograms on a given page and navigation between pages. We discuss the interest of this approach for the evaluation as well as the conception of communicative pictogram grids.