The outside edge of your rhino track is defined as a corner. The rule states you need 8 inches of bumper extending out from every corner.
The 6 inch rule is for if you have a robot shaped like this
http://blogs.solidworks.com/teacher/...-regional.html scroll a few paragraphs down to the picture without the bumper. They have a side of frame less than 8 inches, so they only need 6 inches of bumper, but then they also have 8 additional inches of bumper off of each corner.