The width is not important, its a single reference point. I made this to work things out, a rough copy of what they used in the test. You put it on the seat with the lower left corner touching the base and back of the seat, then put the stick bit at 90 degrees straight up. The string should be level with the ground when measuring the anchor points. They had a bubble that hung on the string.
Mine is marked 700mm for the head rest height, you would need to have a bolt at 450mm for the anchor points.
Here I am measuring my headrest height, the stick goes parallel with the seat back for this one.
Cheers
Stot