3/4 -16 means sixteen threads per inch. Each thread is 1/16" (.062)crest to crest. A half turn of adjustment is 1/32 (.031) off the face of the plug. If you want to move the plug one flat, divide .062 by 8, which equals .008. Eight thousandths off the plug to move one flat.
If you want to fit a half a flat, that's .004 off the plug. It's really easy to take off too much, so try and cut several times before you go for the final fit.