I haven't done my own reflection maps yet, but if I understand it correctly, they should work like other maps. The areas that get reflection are determined by the map. Applying transmaps to your reflection maps, as suggested above, is one way to do it, and another way would be to build a custom reflection map on a template. I haven't used P4 in a long time, but as I remember, there was no way to prevent the invisible areas from showing spectacular.