Many places in openbox that take a coordinate supports a slightly extended format. Most simply it can be just a number such as 300. Such a coordinate will be left- or top-aligned depending on which coordinate it is. To align to the opposite edge, use -300. To specify a negative offset, a - is also used, so you have to use +-10 to offset 10 pixels negatively to the left/top and --10 for the right/bottom edge. Some things enforce being onscreen though, such as the move/resize popup.