cool edge finding trick

from: http://www.practicalmachinist.com/vb/general-archive/best-cheapest-edge-finder-76080/

Here is what I do when I have to pick up an edge dead "**uts" : 

Take a ring gage of known size - any size works that you can get the tip of your indicator inside of. I always grab my .5000.

Stick it down on top of your part or your vise with double sided tape, location does not matter, all you want to do is set your indicator center to that gage. When the indicator is set on zero (rotated the spindle to do this) .. you now know that the center of the spindle is exactly 1/2 of what the size of the ring gage hole is. (.500 ring gage, .2500 off center) 

Now without touching the indicator to knock it out of this position, bring the indicator and spindle over to the edge you are trying to find and sweep the edge. When you see zero on the indicator again you are within (if using the .500 ring gage) .2500 exactly from the edge. 

With a tenth indicator you should be able to pick up this edge within a tenth. This is very reliable, takes all the variables out. 

Of all the electronic center finders and even the "u" shaped finders, I find this to be the most accurate center finder of all. It never lies.