MythTV -- recording ends prematurely?

[gerases]

Just starting to enjoy MythTV on my Ubuntu Dapper. Today, I recorded my
first three shows. The problem is that I think all of them are missing
the last minute or so. For example, I recorded "the Simpsons", which
started at 8 p.m. and it was recorded for 29 minutes and 55 seconds,
but it's supposed to be 30 minutes at least -- right? My clock is good.
Is this a common problem and if so, what's the solution? 



I added 300 seconds in the setup, but is that the best option?


-- 
gerases