Context-awareness is one of the important characteristics for developing dynamic and flexible systems. To develop reliable context-aware systems based on product lines, it is necessary to take into account not only system configurations but also any possible contexts. However, in most cases, contexts are not considered systematically. As a result, unexpected and unfavorable behavior might emerge in a system if a developer does not recognize any possible conflicting combinations between the system and its contexts. Lightweight formal approaches such as VDMTools and Alloy are effective for detecting this kind of conflicts. Because system and context specifications described in these formal methods can be considered as assets, modularity in these specifications is one of the crucial issues. This paper proposes an assessment framework for selecting appropriate formal approaches in terms of the modularity.