Did you mean the setting triangles next to the border?? You want to make sure that the outside edge is straight of grain. One of the most popular way of cutting them is cutting from strips using a similar ruler:
http://www.ebay.com/itm/DIAGONAL-SET...item33a10c273f
The old fashion way was to cut a larger square out then do two diagonal cuts which would produce 4 setting triangles. I think the formula was 1.5 x size of your block. Not good with math anymore.