There is an answer which would be helpful and measurable. Since OP has his head stuck in a calculator it might be useful. Otherwise N1150x has a very good answer.
Where continued adjustments would only move the Cartesian coordinates for POI from one quadrant to another without decreasing the distance from actual POA to the actual POI. After you adjust your turrets the first time you should be able to drop in another shot and if that shot hits within your normal group size of the previous shot, regardless of direction, you're zero'd. Continuing beyond that is an utter W.O.M.B.A.T. (Waste of money, brains and time.)
Mathematically, I guess when you can execute a shot group and 95% of the time average POI is within 2 standard deviations of your shot group size for the rifle/load combo from POA you can call yourself zero'd.
You know, I have a page on my spreadsheet that's meant to help people through this sort of unnecessary academic putting from the rough. http://ballisticxlr.com, download (free), use the statistics tab. If you need help, ping me at
[email protected].