Speaking of Tattenham Corner and MJTs Mike, some YouTubers recently used Oyster to make a 2h45m journey from Chesham to Tattenham Corner on a weekday (0430-1900).
As per the MJT tables on your site it’s a 14-zone journey for which the limit is 3h20m, however the TfL guidance (which the YouTubers referred to) classes it as an 11-zone limit of 2h50m and basically says you get no extra time for any travel beyond Zone 6. What’s up with that?
(N.B. I’m guessing you obtained the extended tables through a FoI request)