I have seen two methods for calculating retention. The easiest is to weigh the beans going in and the grounds coming out. However, that seems flawed in that you don't know what is coming out. Some of your fresh beans remain in the grinder and some of what comes out was there before. In fact, my Mignon Specialita sometimes get more out (by a half gram or so than I put in. I suspect it is not manufacturing coffee so that is unnerving. Mixing is also noticeable if I make a shot of decaf in the evening and feel .... wired. The day's standard beans are still getting into the shot, even though the output is almost exactly the same as the input. In/out is a meaningless specification as far as I can see and my little Mignon does retain more than I would like.
A much more tedious method is to clean the grinder, put through a number of doses and then sweep out everything left and weigh that. This seems to be a more reasonable measure of retention, but it is rarely done. Does anyone have a retention spec that is easier than the clean/residue method but more meaningful than in/out?
Or I guess we avoid the issue and purge - which I find offends my ingrained aversion to wasting food - or go with something designed ground up for single dosing. Expensive.