There is no set formula to translate. It depends on the relative strength of your stabilizer muscles. Someone with underdeveloped triceps but a powerful chest, for example, will move more weight with the barbell than the dumbbells because the triceps are one of the stabilizers.

Someone with weak shoulders will also suffer in the dumbbell movement, and increase their risk of injury with the barbell. This is why both are actually important in a routine - the dumbbell for the unilateral isolation of various muscles to allow adequate loading/coordination response, and the barbell to increase the loading directly on the chest by taking out some of the synergists and stabilizers as weak links.