The 31% to 33% may have been from 30.5% to 33.5% and depending on the bitrate of the programme recorded, this estimate is not that far off. Record some more/longer programmes and the % will probably be accurate. Here's the link to the bitrate of various channels:
The lower bitrate channels will use only about 5 GB/hour, so if you recorded mostly those channels, you'd approach 100 hours with 500 GB. If you recorded only the higher bitrate HD channels (about 8 GB/hour), then you'd get the 60 hours.