I have a total output of, may be, 2 instruments a year right now, and that is a good year. This year it will be one instrument, a cnc router and a cyclone vacuum system (an exceptionally good year). I don't have enough of an output to do any kind of statistics on.
But I have worked with SPC in my day job and highly recomend it assuming you have the amount of output so it will give you useful information.
If you cut it to small you can always nail another piece on the end, but if you cut it to big... then what the hell you gonna do?
Steven