A central objective of petrogenetic studies is to determine the proportions of crystallised magma to assimilated country-rock. In assimilation-fractional crystallisation (AFC) calculations, this is represented by the “r-value”. To calculate assimilation/crystallisation proportions, it is necessary to know the compositions of both the starting magma and the assimilant end-members. For a single assimilant, these calculations are relatively straightforward. However, geology is rarely so simple. How does one quantify the percentage of crustal assimilation involved with magma assembly when multiple assimilants are involved? Using an example from Tongariro volcano, New Zealand, this talk will discuss some of the challenges with (1) accurately identifying crustal assimilants and (2) determining their contributions to arc magma volumes.