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Mobile applications require an adaptation phase so as to adapt to the user context. Utility functions or rules are most often used to make the adaptation planning or decision (i.e. select the best adapted variant for each required service). Fuzzy controllers are used when it is difficult or even impossible to construct precise mathematical models. In the case of mobile applications, the large number of Quality of Service (QoS) and context parameters causes rule number explosion. An exponential increase in the number of rules increases the processing time of the adaptation planning. To reduce the processing time and simplify the fuzzy control system, we propose the concept of ideal QoS model. With it, fuzzy values of ideal QoS parameters are calculated using a fuzzy control system to fit the context state and user preferences. A fuzzy logic similarity method based on fuzzy sets and fuzzy operators is proposed to select among all service variants, the variant having the nearest QoS values to the ideal. Experiments show that our approach can significantly improve both the processing time and the number of rules when selecting the variant that well adapts to environment changes.