In TBHs you don't have the same chimney effect you would get from a tall stack of Langstroth boxes, so the position of the entrances is not so important. We put three one-inch holes at each end of the box, on the long side of the box. That allows you to open up the far end from the brood nest when the population of the hive is really large. Nectar foragers can enter there without having to travel through the brood nest.
Our entrance holes are two inches off the bottom, but I doubt that it's important exactly where they are. Not having them at the top may conserve some heat in winter, but we used open screens on the bottom so the effect is probably small.