resize object question

I would like to reduce the size of an object in Keynote by 1/2 inch. How do I convert pts to inches. I tried to search for this info, but I could only find 72 pts equals an inch, which is correct when dealing with fonts. I need to know what the points that Keynote uses for sizing objects. It is not the same as font points because keynote shows my object is 539 pts wide; however, it measures only a little more than 5 1/2 inches wide.

Unfortunately the "object" I want to reduce is a grouping of objects, so when I click on it, the dashed-line border indicating the edges of the object is larger than the actual "image" I am interested in. So I can only measure the actual image and guesstimate the white area between edge of the image and Keynote's edge pf object. If they were the same I could calculate my own answer. And I can "get there" with brute force by continuing to resize and measure, but I would prefer not to waste printer ink.

And I would like to know for future reference as well. This must be standard. Can someone please enlighten me?