Product lines (PL) modeling have proven to be an effective approach to reuse in software development. Several variability approaches were developed to plan requirements reuse, but only little of them actually address the issue of deriving product requirements. This paper presents a method, RED-PL that intends to support requirements derivation. The originality of the proposed approach is that (i) it is user-oriented, (ii) it guides product requirements elicitation and derivation as a decision making activity, and (iii) it provides systematic and interactive guidance assisting analysts in taking decisions about requirements. The RED-PL methodological process was validated in an industrial setting by considering the requirement engineering phase of a product line of blood analyzers.