The maximum value of the ratio between the signal gain per unit observing time for fixed field of view to the same for fixed observing depth, calculated for various telescope fields of view and limiting magnitudes. All values are calculated with reference to a Lyman-α-selected galaxy survey with spectroscopic redshifts used for cross-correlation with a 21-cm measurement from SKA-low, assuming that 21-cm foregrounds extend to the horizon. Values greater than one would indicate that increasing the observing depth yields more signal than increasing the field of view, for the same amount of time spent. All values in the domain shown are less than one, indicating that increasing the field of view yields more signal per unit time than increasing the observing depth.