[mythtv-users] when is a mythfrontend preview captured?

[Roger Siddons]

Looks like it was changed in 0.24 to be a 1/3 of the way in, which is 
close-ish to your figures.

https://github.com/MythTV/mythtv/commit/edb680fc9c496348ff77f2d264a9c42add665983