Prudence would suggest more reserve, especially if other factors (cloud, local hills/escarpments - I'm looking at you, YWOL, etc come into play) but I was asking about the understanding of the law, not what was taught to students.
(A personal mistake made with this involved an overwater crossing at 4000ft for glide safety, followed by a misjudged descent and spending 5 minutes circling to descend, during which time it got very dark indeed. My ETA at the destination was 20 minutes before last light, and I was within the vicinity of the circuit by that time, but actually getting on the ground took more than I budgeted for. I'm not making that mistake again.)