I was given the clearance "descend now FL240" a couple of weeks ago and i read back "descend now 240", immediately and after thinking about it, quite rightly,i was asked to confirm flight Level 240,which at the time i was convinced i already had,but I could have read back "descend to 40",big difference.I used to fly with a guy (old school) who said that " The words TO and FOR should never be used on the RT,he has a point!