Didn't read your link... however, if you have lots of mail in sent and inbox, create a new mailbox on your drive, (a folder) and in it --- two more: one in, one sent
Drag your .mac email to their respective folders. They will now be on your HD, not on your .mac account. Also, empty your .mac trash.
that will clear your idisk of excess. This is assuming you have nothing else stored on that
disk.