As most of you know, I have been harvesting .999 canadian nickels for the best part of three years now. I have been keeping track of my finds including percentages. My 20 boxes moving average has now dropped to 8% and has been steadily going lower and lower. I am at box 214 (only 36 to go for the half million nickel sorted) so I expect that by the time (sometimes this summer-fall) I reach my goal, that the average box will yield below 7.5%.
Past this point, it is no longer logical to keep stacking on a regular basis.