That's good, but having said that I have had a bit of a thought about this and I think the +/- 0.5 stones is probably too tight, and possibly too difficult to get! There is no point in having a mysterious one if there is nobody to notice the mystery! My instinct tells me that there ought to be some small sensible tolerance - the same across all MM targets to keep things simple - and I think it probably ought to be either +/- 10 stones, or perhaps 0.1% of the relevant MM (i.e. plus OR minus 0.1%) if that is easy to do.
I feel that this way people would see a few more of them and wonder how they got something for reaching 999,001!!! If anything it would probably add to the mystique. What do you think?