Lets call one variant the wild type and the other the mutant .
Virus need not be growing exponentially during the experiment. The formula uses the fold expansion of the wild type virus, and the fold change in the ratio of the genotype frequencies. This is all computed for you if you provide the total virus concentrations, and the percentages of mutant virus, at the start and at the end of the experiment. In case you refresh during the experiment, i.e. dilute the productively infected cells, provide the dilution factor. In case you have no clue, you may enter a zero.