Graph indices are a key to manage huge amounts of distributed graph data. Instance-level indices are available that focus on the fast retrieval of nodes. Furthermore, there are so-called schema-level indices focusing on summarizing nodes sharing common characteristics, i. e., the combination of attached types and used property-labels. We argue that there is not a one-size-fits-all schema-level index. Rather, a parameterized, formal model is needed that allows to quickly design, tailor, and compare different schema-level indices. We abstract from related works and provide the formal model FLuID using basic building blocks to flexibly define different schema-level indices. The FLuID model provides parameterized simple and complex schema elements together with four parameters. We show that all indices modeled in FLuID can be computed in O(n). Thus, FLuID enables us to efficiently implement, compare, and validate variants of schema-level indices tailored for specific application scenarios.