> Say you've got a 100g drive with a 40g database and for purposes of discussion the backup is also 40g, leaving you a cool 20g free. Add 10% to the db driving the size up to 44g, you now only have 12g free after backup - and that's not counting your log backups.

Just a thought... one might consider using a different example, if the drive itself fails and is lost what of the backup?