Pen-based interaction is well adapted for writing down information in a mobile context. However, there is a lack of software taking advantage of this interaction process to design technical documents in constrained environments. This is because sketch interpretation is a complex research problem and good performances are required to design industrial software. The first contribution of this article is to show how "on-the-fly" interpretation is a viable approach to design robust and efficient sketch interpretation systems. With this process, the system interprets each stroke after its drawing and produces a visual feedback to the user. The interaction with the user is then strongly taken into account to get pertinent information and better interpret the document. Moreover, on-the-fly interpretation opens a large panel of possibilities to design accurate documents, such as contextual help, complex layout reproduction, etc. Of course, such a process has several requirements to be usable in practice, for example to avoid disturbing the user with inconsistent feedback. The second contribution of this paper is a set of strategies, based on grammatical modeling and pattern recognition, to face these needs. We present two industrial pen-based software that are based on the principles we present in this article. One of them, dedicated to architectural floor-plan design, as not been presented previously.