In DS 4.7 the Render Settings dialogue can do the maths for you - just type 8.5300 and 11300, for example, in the dimension boxes. A DS render doesn't have a PPI setting -that's just a number at the front of the file saying how big a pixel should be when printed, or placed in a document specified in real-world units. Most editors and viewers will use either 72PPI or 96PPI for untagged images.