Poole et al. (2008) provide the conversion factors between count rate and fluxes, as well as the zero-point magnitudes. Using C = 10[(ZPT-m)/2.5], where C = count rate, ZPT = zero-point value and m = magnitude, the above plot can be used to estimate the magnitudes at which the grisms are useful. This method shows that magnitude 9-10 is the brightest useful source for the UV grism (about magnitude 11-12 for the V grism).
The grism can be useful for sources with UV fluxes as low as ~10-15 erg cm-2 s-1 A-1 (around magnitude 16).