Health care demands rigour in the application and quality of treatment practices. The accuracy of dose calculations during courses of medication engages the responsibility of nurses. The mathematical operations stipulated in the procedures are complex. In emergency situations, errors can be committed that have tragic consequences for patients. To reinvest formal knowledge learned in training in a professional situation, the practitioner has to conduct intellectual operations which must make the pre-established categories of knowledge match appropriately all the characteristics of the situation encountered. The Authors looked into how professionals develop their dose calculation skills. Tests were presented to 60 students and interviews held with 15 experienced nurses. The established practitioners appeared to cultivate two operational perspectives: one acquired in training (a command of the mathematical principles) and the other gained through working, sometimes with no interconnection between the two. They move away from the model taught in training, to construct “practical rules” which  they find better adapted for effective use of their knowledge for practical working situations.