Need a external HD 40% bigger than internal for Time Machine backups??

I've heard that using Time Machine as my backup program I need a hard drive that's about 40% bigger than the source (internal) hard drive I want to backup. Some in these forums have said I need a external HD 2x or 3x as big as the internal hard drive for Time Machine backups!
What's your general rule, and does Apple recommend a certain percentage increase of the target over the source hard drive?

There is no set rule, except that the drive being used for Time Machine must be at least as large as the data (not drive) being backed up.
So if you have an internal 500GB drive, but you only have 300GB full, and with exclusions (things that you select NOT to back up), only 200GB needs to be backed up, then a 320GB external drive could do an adequate job (even though the internal drive is 500GB).
During the first backup, Time Machine will back up everything (that is everything you have set it to backup). So the external drive volume used for the backup must be at least the size of the data being backed up. After that, Time Machine creates an incremental backup states every hour, for things that changed during that hour. But because it is Time Machine and not just Backup Machine, the previous versions of the files that changed are also retained. Therefore, over time, the Time Machine backup archive will take up more and more space.
Once you run out of space, Time Machine will tell you and start deleting the oldest backup states to make room for the newest states. So if you have a really huge volume for your Time Machine backup, you can +go back in time+ further (probably back to the date you started using Time Machine) than if you have an adequate but smaller size drive for that purpose.

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