The fluorescence lifetime τ enters into the Perrin-Weber equation used to determine rotational relaxation times of proteins by the steady-state depolarization method. If is either too short or too long, the relaxation time cannot be measured with precision. With reasonable assumptions as to photometric precision, it has been possible to compute the precision of relaxation time assays for different lifetime:relaxation time ratios, thus also determining the “optimum” lifetimes for different systems. These results provide a basis for designing experiments requiring optimal conditions, and also provide insight into the amount of allowable deviation from optimal conditions. The short lifetimes of intrinsic protein fluorescence can often be used to determine the global relaxation rate of the macromolecule with acceptable precision.