A single guideline is all that is needed to locate the endpoints of an arc when radiusing the corners of a rectangle or square. SketchUp's inferencing will handle the rest.
After the first corner is complete, the same thing can be done at the opposite edge where the guideline crosses. To locate the arcs for the other two corners, hover over the end of the corresponding arc, in the clip above nearest the green axis. Then move the cursor to the other side of the rectangle and move along the edge until a dotted line appears connecting the first arc with the end point for the new arc.
