It depends (doesn’t everything!)
If I’m trying to fit it onto a piece of material that’s just big enough I’ll do it multiple times around all the edges to make sure none of them fall over (remember the view is only for the last place you did Set Focus, so don’t adjust something on the other side based on this side.)
If I’m doing something in the corner of a brand new sheet, I just do as far right and down as I can get so it’s closest to the edge.
If you want to see if you can get even closer than your 1/8" (some of us have gotten to sub mm) you can try running the Camera Calibration. Keep in mind that there are no guarantees beyond the 1/4", but I found it to be worth it 
Once you’ve run it, you should never need to do so again unless you move your machine or something major.
Also - it says use a clean sheet of proofgrade, what it needs is a flat piece of anything. Many have found that a nice piece of cardboard coated with paper masking (so it’s a solid white) does just as well.
This is my favourite picture (belongs to @rbtdanforth) of how accurate this machine can be
