On Fri, 7 Nov 2008 07:42:29 -0800 (PST), martin j wrote:

Got to get to Harrow on the Hill, Tuesday evening and Journey Planner
says 12 mins for the Met Line from Finchley Road to Harrow on the
Hill.
How much should I add to get a realistic figure?
Many thanks
martin J

Depends what time of the evening, IMX. At around 5-6pm it's very fast,
frequent, and reliable. However, after around 6pm, reliability
plummets, (relatively) large gaps appear in the service, everything
starts stopping at Wembley Park, and many of the Watford trains become
all-stations, so I'd allow at least an extra 7 minutes.