I ... have to add HOURS to the padding to get the end of shows, (at the moment 20 minutes) that fills the hard disk of the recorder with unwanted material.
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HOURS is much more than you usually need. Except for breaking news days, the scheduling games the networks play don't mess with the schedules nearly that much. The worst I've seen was a Big Brother final that ran about 50 minutes over (some time ago, so long ago that Gretel Killeen was still hosting). But that's quite unusual.
For the commercial networks' prime time, the main evening news will start on time, but by the time the news and any following current affairs programming has run, they'll no longer be running to their published print schedule.
I've got by with 5 min pre-padding and 25-min post padding for a long time now, with very few losses of the end of programs. Because IceTV rounds their program start and end times to the nearest 5 minutes, I don't think that Dave's suggestion of 3 minutes pre-padding is enough.
What can cause problems is when you have a crowded recording schedule and padding can't be added to all the recordings, because the available recording slots are taken up by other recordings starting up.
If you have a 2-tuner PVR, and you set, say, ABC1 to record 20:00-20:30, SBS1 to record 20:30-21:00 and Nine to record 20:00-21:00, the ABC1 and SBS1 recordings can only have padding at the expense of the other recording.