You can trace the nets as they follow and then use the tool to spread them evenly.
In Expedition EE7.9.x you use the Smart utility (Make sure they are installed).
In VX.1 they are installed automatically, and are placed under the Route menu.
In the example of VX.1 you would use Route -> Align Trace -> Center align -> Equal spacing between object. Select the two most outward traces and let the utility spread the traces evenly in between.
Hope this helps.
...or use rule areas with increasing trace-to-trace minimum spacing. Avoid enclosing the diagonal area though, enclose only the orthogonal.