As well as understanding the location of dopants in optical materials, it is also important to understand how much dopant can be added to a given material. A method for calculating the maximum concentration of dopants has been developed, and applied to dopants in mixed metal fluorides for optical and nuclear clock applications. Applications to rare earth doping in YLiF 4 , and Th doping in LiCaAlF 6 /LiSrAlF 6 are described, and compared with available experimental data.