The 0.2 or 0.25 (assuming the lens has no loss due to focusing closer than infinity, object not at center, lens flare etc...) with the condition that the subject reflects 100% the light incident on it. If the subject is a gray card (or some kind of average scene) then it is 0.18 x 0.25 = 0.045 and that is -4.47 stops less.