Measuring the effectiveness of your redirects

When I work on a site migration I like to redirect everything I possibly can find, very much in a 'no one gets left behind' sort of way. Most migration projects don't do this, for a multitude of reasons, but years ago I was told to only redirect the top 200 pages of a site, which was about 10% of the live pages, because it was not worth the time and effort to redirect the other urls. I did…