you can't unless you turn the ruler side ways and mark the amount to cut and then line up he ruler.
There are rulers that are marked 1/8" increments.
You might consider cutting at 7" and taking a slightly larger seam when you sew the triangles together. Or make the square larger and then cut dow to size when the two triangles are sewed together. That is if you are cutting triangles and then sewing into squares.
Or get a ruler that has 1/8" marks on it. Or use the 6 1/2 ruler and then with another ruler measure the additional and mark and then cut. I have on occasion cut a narrow strip of stiff plastic and taped to one side of the ruler to give the additional inches and then use that side to line up and the un taped side for the cutting.